new terms Flashcards
.ARM FILE
Metadata for ARCHIVE tables. Contrast with .ARZ file. Files with this extension are always included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
.ARZ file
Data for ARCHIVE tables. Contrast with .ARM file. Files with this extension are always included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
.cfg file
A metadata file used with the InnoDB transportable tablespace feature. It is produced by the command FLUSH TABLES … FOR EXPORT, puts one or more tables in a consistent state that can be copied to another server. The .cfg file is copied along with the corresponding .ibd file, and used to adjust the internal values of the .ibd file, such as the space ID, during the ALTER TABLE … IMPORT TABLESPACE step.
.frm file
REMOVED IN MYSQL 8.0
A file containing the metadata, such as the table definition, of a MySQL table.
For backups, you must always keep the full set of .frm files along with the backup data to be able to restore tables that are altered or dropped after the backup.
Although each InnoDB table has a .frm file, InnoDB maintains its own table metadata in the system tablespace; the .frm files are not needed for InnoDB to operate on InnoDB tables.
These files are backed up by the MySQL Enterprise Backup product. These files must not be modified by an ALTER TABLE operation while the backup is taking place, which is why backups that include non-InnoDB tables perform a FLUSH TABLES WITH READ LOCK operation to freeze such activity while backing up the .frm files. Restoring a backup can result in .frm files being created, changed, or removed to match the state of the database at the time of the backup.
.ibd file
Each InnoDB table created using the file-per-table mode goes into its own tablespace file, with a .ibd extension, inside the database directory. This file contains the table data and any indexes for the table. File-per-table mode, controlled by the innodb_file_per_table option, affects many aspects of InnoDB storage usage and performance, and is enabled by default in MySQL 5.6.7 and higher.
This extension does not apply to the system tablespace, which consists of the ibdata files.
When a .ibd file is included in a compressed backup by the MySQL Enterprise Backup product, the compressed equivalent is a .ibz file.
If a table is create with the DATA DIRECTORY = clause in MySQL 5.6 and higher, the .ibd file is located outside the normal database directory, and is pointed to by a .isl file.
.ibz file
When the MySQL Enterprise Backup product performs a compressed backup, it transforms each tablespace file that is created using the file-per-table setting from a .ibd extension to a .ibz extension.
The compression applied during backup is distinct from the compressed row format that keeps table data compressed during normal operation. A compressed backup operation skips the compression step for a tablespace that is already in compressed row format, as compressing a second time would slow down the backup but produce little or no space savings.
.isl file
This metadata file has been removed in 8.0
A file that specifies the location of a .ibd file for an InnoDB table created with the DATA DIRECTORY = clause in MySQL 5.6 and higher. It functions like a symbolic link, without the platform restrictions of the actual symbolic link mechanism. You can store InnoDB tablespaces outside the database directory, for example, on an especially large or fast storage device depending on the usage of the table. For details, seeSection 14.5.4, “Specifying the Location of a Tablespace”.
.MRG file
A file containing references to other tables, used by the MERGE storage engine. Files with this extension are always included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
.MYD file
A file that MySQL uses to store data for a MyISAM table.
.MYI file
A file that MySQL uses to store indexes for a MyISAM table.
.OPT file
A file containing database configuration information. Files with this extension are always included in backups produced by the mysqlbackupcommand of the MySQL Enterprise Backup product.
.PAR file
This metadata file has been removed in 8.0
A file containing partition definitions. Files with this extension are included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
With the introduction of native partitioning support for InnoDB tables in MySQL 5.7.6, .par files are no longer created for partitioned InnoDB tables. Partitioned MyISAM tables continue to use .par files in MySQL 5.7. In MySQL 8.0, partitioning support is only provided by the InnoDB storage engine. As such, .par files are no longer used as of MySQL 8.0.
.TRG file
This metadata file has been removed in 8.0
A file containing trigger parameters. Files with this extension are always included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
.TRN file
This metadata file has been removed in 8.0
A file containing trigger namespace information. Files with this extension are always included in backups produced by the mysqlbackupcommand of the MySQL Enterprise Backup product.
ACID
An acronym standing for atomicity, consistency, isolation, and durability. These properties are all desirable in a database system, and are all closely tied to the notion of a transaction. The transactional features of InnoDB adhere to the ACID principles.
Transactions are atomic units of work that can be committed or rolled back. When a transaction makes multiple changes to the database, either all the changes succeed when the transaction is committed, or all the changes are undone when the transaction is rolled back.
The database remains in a consistent state at all times – after each commit or rollback, and while transactions are in progress. If related data is being updated across multiple tables, queries see either all old values or all new values, not a mix of old and new values.
Transactions are protected (isolated) from each other while they are in progress; they cannot interfere with each other or see each other’s uncommitted data. This isolation is achieved through the locking mechanism. Experienced users can adjust the isolation level, trading off less protection in favor of increased performance and concurrency, when they can be sure that the transactions really do not interfere with each other.
The results of transactions are durable: once a commit operation succeeds, the changes made by that transaction are safe from power failures, system crashes, race conditions, or other potential dangers that many non-database applications are vulnerable to. Durability typically involves writing to disk storage, with a certain amount of redundancy to protect against power failures or software crashes during write operations. (In InnoDB, the doublewrite buffer assists with durability.)
adaptive flushing
An algorithm for InnoDB tables that smooths out the I/O overhead introduced by checkpoints. Instead of flushing all modified pages from the buffer pool to the data files at once, MySQL periodically flushes small sets of modified pages. The adaptive flushing algorithm extends this process by estimating the optimal rate to perform these periodic flushes, based on the rate of flushing and how fast redo information is generated. First introduced in MySQL 5.1, in the InnoDB Plugin.
adaptive hash index
An optimization for InnoDB tables that can speed up lookups using = and IN operators, by constructing a hash index in memory. MySQL monitors index searches for InnoDB tables, and if queries could benefit from a hash index, it builds one automatically for index pages that are frequently accessed. In a sense, the adaptive hash index configures MySQL at runtime to take advantage of ample main memory, coming closer to the architecture of main-memory databases. This feature is controlled by the innodb_adaptive_hash_index configuration option. Because this feature benefits some workloads and not others, and the memory used for the hash index is reserved in the buffer pool, typically you should benchmark with this feature both enabled and disabled.
The hash index is always built based on an existing InnoDB secondary index, which is organized as a B-tree structure. MySQL can build a hash index on a prefix of any length of the key defined for the B-tree, depending on the pattern of searches against the index. A hash index can be partial; the whole B-tree index does not need to be cached in the buffer pool.
In MySQL 5.6 and higher, another way to take advantage of fast single-value lookups with InnoDB tables is to use the memcached interface to InnoDB. See Section 14.18, “InnoDB Integration with memcached” for details
AHI
Acronym for adaptive hash index.
AIO
Acronym for asynchronous I/O. You might see this acronym in InnoDB messages or keywords.
Antelope
The code name for the original InnoDB file format. It supports the redundant and compact row formats, but not the newer dynamic and compressed row formats available in the Barracuda file format.
If your application could benefit from InnoDB table compression, or uses BLOBs or large text columns that could benefit from the dynamic row format, you might switch some tables to Barracuda format. You select the file format to use by setting the innodb_file_format option before creating the table.
application programming interface (API)
A set of functions or procedures. An API provides a stable set of names and types for functions, procedures, parameters, and return values.
apply
When a backup produced by the MySQL Enterprise Backup product does not include the most recent changes that occurred while the backup was underway, the process of updating the backup files to include those changes is known as the apply step. It is specified by the apply-log option of the mysqlbackup command.
Before the changes are applied, we refer to the files as a raw backup. After the changes are applied, we refer to the files as a prepared backup. The changes are recorded in the ibbackup_logfile file; once the apply step is finished, this file is no longer necessary.
asynchronous I/O
A type of I/O operation that allows other processing to proceed before the I/O is completed. Also known as non-blocking I/O and abbreviated as AIO. InnoDB uses this type of I/O for certain operations that can run in parallel without affecting the reliability of the database, such as reading pages into the buffer pool that have not actually been requested, but might be needed soon.
Historically, InnoDB has used asynchronous I/O on Windows systems only. Starting with the InnoDB Plugin 1.1 and MySQL 5.5, InnoDB uses asynchronous I/O on Linux systems. This change introduces a dependency on libaio. Asynchronous I/O on Linux systems is configured using the innodb_use_native_aio option, which is enabled by default. On other Unix-like systems, InnoDB uses synchronous I/O only.
atomic ddl
An atomic DDL statement is one that combines the data dictionary updates, storage engine operations, and binary log writes associated with a DDL operation into a single, atomic transaction. The transaction is either fully committed or rolled back, even if the server halts during the operation. Atomic DDL support was added in MySQL 8.0. For more information, see Section 13.1.1, “Atomic Data Definition Statement Support”.
atomic
In the SQL context, transactions are units of work that either succeed entirely (when committed) or have no effect at all (when rolled back). The indivisible (“atomic”) property of transactions is the “A” in the acronym ACID.
atomic instruction
Special instructions provided by the CPU, to ensure that critical low-level operations cannot be interrupted.
auto-increment
A property of a table column (specified by the AUTO_INCREMENT keyword) that automatically adds an ascending sequence of values in the column. InnoDB supports auto-increment only for primary key columns.
It saves work for the developer, not to have to produce new unique values when inserting new rows. It provides useful information for the query optimizer, because the column is known to be not null and with unique values. The values from such a column can be used as lookup keys in various contexts, and because they are auto-generated there is no reason to ever change them; for this reason, primary key columns are often specified as auto-incrementing.
Auto-increment columns can be problematic with statement-based replication, because replaying the statements on a slave might not produce the same set of column values as on the master, due to timing issues. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb_autoinc_lock_mode=1. If you have innodb_autoinc_lock_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. The setting innodb_autoinc_lock_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.
auto-increment locking
The convenience of an auto-increment primary key involves some tradeoff with concurrency. In the simplest case, if one transaction is inserting values into the table, any other transactions must wait to do their own inserts into that table, so that rows inserted by the first transaction receive consecutive primary key values. InnoDB includes optimizations, and the innodb_autoinc_lock_mode option, so that you can choose how to trade off between predictable sequences of auto-increment values and maximum concurrency for insert operations.
autocommit
A setting that causes a commit operation after each SQL statement. This mode is not recommended for working with InnoDB tables withtransactions that span several statements. It can help performance for read-only transactions on InnoDB tables, where it minimizes overhead from locking and generation of undo data, especially in MySQL 5.6.4 and up. It is also appropriate for working with MyISAM tables, where transactions are not applicable.
availability
The ability to cope with, and if necessary recover from, failures on the host, including failures of MySQL, the operating system, or the hardware and maintenance activity that may otherwise cause downtime. Often paired with scalability as critical aspects of a large-scale deployment.
B-tree
A tree data structure that is popular for use in database indexes. The structure is kept sorted at all times, enabling fast lookup for exact matches (equals operator) and ranges (for example, greater than, less than, and BETWEEN operators). This type of index is available for most storage engines, such as InnoDB and MyISAM.
Because B-tree nodes can have many children, a B-tree is not the same as a binary tree, which is limited to 2 children per node.
Contrast with hash index, which is only available in the MEMORY storage engine. The MEMORY storage engine can also use B-tree indexes, and you should choose B-tree indexes for MEMORY tables if some queries use range operators.
backticks
Identifiers within MySQL SQL statements must be quoted using the backtick character () if they contain special characters or reserved words. For example, to refer to a table named FOO#BAR or a column named SELECT, you would specify the identifiers as FOO#BAR and SELECT`. Since the backticks provide an extra level of safety, they are used extensively in program-generated SQL statements, where the identifier names might not be known in advance.
Many other database systems use double quotation marks (“) around such special names. For portability, you can enable ANSI_QUOTES mode in MySQL and use double quotation marks instead of backticks to qualify identifier names.
backup
The process of copying some or all table data and metadata from a MySQL instance, for safekeeping. Can also refer to the set of copied files. This is a crucial task for DBAs. The reverse of this process is the restore operation.
With MySQL, physical backups are performed by the MySQL Enterprise Backup product, and logical backups are performed by the mysqldump command. These techniques have different characteristics in terms of size and representation of the backup data, and speed (especially speed of the restore operation).
Backups are further classified as hot, warm, or cold depending on how much they interfere with normal database operation. (Hot backups have the least interference, cold backups the most.)
Barracuda
The code name for an InnoDB file format that supports compression for table data. This file format was first introduced in the InnoDB Plugin. It supports the compressed row format that enables InnoDB table compression, and the dynamic row format that improves the storage layout for BLOB and large text columns. You can select it through the innodb_file_format option.
Because the InnoDB system tablespace is stored in the original Antelope file format, to use the Barracuda file format you must also enable the file-per-table setting, which puts newly created tables in their own tablespaces separate from the system tablespace.
The MySQL Enterprise Backup product version 3.5 and above supports backing up tablespaces that use the Barracuda file format.
base column
A non-generated table column upon which a stored generated column or virtual generated column is based. In other words, a base column is a non-generated table column that is part of a generated column definition.
beta
An early stage in the life of a software product, when it is available only for evaluation, typically without a definite release number or a number less than 1. InnoDB does not use the beta designation, preferring an early adopter phase that can extend over several point releases, leading to a GA release.
binary log
A file containing a record of all statements that attempt to change table data. These statements can be replayed to bring slave servers up to date in a replication scenario, or to bring a database up to date after restoring table data from a backup. The binary logging feature can be turned on and off, although Oracle recommends always enabling it if you use replication or perform backups.
You can examine the contents of the binary log, or replay those statements during replication or recovery, by using the mysqlbinlog command. For full information about the binary log, see Section 5.2.4, “The Binary Log”. For MySQL configuration options related to the binary log, see Section 17.1.4.4, “Binary Log Options and Variables”.
For the MySQL Enterprise Backup product, the file name of the binary log and the current position within the file are important details. To record this information for the master server when taking a backup in a replication context, you can specify the –slave-info option.
Prior to MySQL 5.0, a similar capability was available, known as the update log. In MySQL 5.0 and higher, the binary log replaces the update log.
binlog
An informal name for the binary log file. For example, you might see this abbreviation used in e-mail messages or forum discussions.
blind query expansion
A special mode of full-text search enabled by the WITH QUERY EXPANSION clause. It performs the search twice, where the search phrase for the second search is the original search phrase concatenated with the few most highly relevant documents from the first search. This technique is mainly applicable for short search phrases, perhaps only a single word. It can uncover relevant matches where the precise search term does not occur in the document.
BLOB
An SQL data type (TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB) for objects containing any kind of binary data, of arbitrary size. Used for storing documents, images, sound files, and other kinds of information that cannot easily be decomposed to rows and columns within a MySQL table. The techniques for handling BLOBs within a MySQL application vary with each Connector and API. MySQL Connector/ODBC defines BLOB values as LONGVARBINARY. For large, free-form collections of character data, the industry term is CLOB, represented by the MySQL TEXT data types.
bottleneck
A portion of a system that is constrained in size or capacity, that has the effect of limiting overall throughput. For example, a memory area might be smaller than necessary; access to a single required resource might prevent multiple CPU cores from running simultaneously; or waiting for disk I/O to complete might prevent the CPU from running at full capacity. Removing bottlenecks tends to improve concurrency. For example, the ability to have multiple InnoDB buffer pool instances reduces contention when multiple sessions read from and write to the buffer pool simultaneously.
bounce
A shutdown operation immediately followed by a restart. Ideally with a relatively short warmup period so that performance and throughput quickly return to a high level.
buddy allocator
A mechanism for managing different-sized pages in the InnoDB buffer pool.
buffer
A memory or disk area used for temporary storage. Data is buffered in memory so that it can be written to disk efficiently, with a few large I/O operations rather than many small ones. Data is buffered on disk for greater reliability, so that it can be recovered even when a crash or other failure occurs at the worst possible time. The main types of buffers used by InnoDB are the buffer pool, the doublewrite buffer, and the insert buffer.
buffer pool
The memory area that holds cached InnoDB data for both tables and indexes. For efficiency of high-volume read operations, the buffer pool is divided into pages that can potentially hold multiple rows. For efficiency of cache management, the buffer pool is implemented as a linked list of pages; data that is rarely used is aged out of the cache, using a variation of the LRU algorithm. On systems with large memory, you can improve concurrency by dividing the buffer pool into multiple buffer pool instances.
Several InnoDB status variables, information_schema tables, and performance_schema tables help to monitor the internal workings of the buffer pool. Starting in MySQL 5.6, you can also dump and restore the contents of the buffer pool, either automatically during shutdown and restart, or manually at any time, through a set of InnoDB configuration variables such as innodb_buffer_pool_dump_at_shutdown and innodb_buffer_pool_load_at_startup.
buffer pool instance
Any of the multiple regions into which the buffer pool can be divided, controlled by the innodb_buffer_pool_instancesconfiguration option. The total memory size specified by the innodb_buffer_pool_size is divided among all the instances. Typically, multiple buffer pool instances are appropriate for systems devoting multiple gigabytes to the InnoDB buffer pool, with each instance 1 gigabyte or larger. On systems loading or looking up large amounts of data in the buffer pool from many concurrent sessions, having multiple instances reduces the contention for exclusive access to the data structures that manage the buffer pool.
built-in
The built-in InnoDB storage engine within MySQL is the original form of distribution for the storage engine. Contrast with the InnoDB Plugin. Starting with MySQL 5.5, the InnoDB Plugin is merged back into the MySQL code base as the built-in InnoDB storage engine (known as InnoDB 1.1).
This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.
business rules
The relationships and sequences of actions that form the basis of business software, used to run a commercial company. Sometimes these rules are dictated by law, other times by company policy. Careful planning ensures that the relationships encoded and enforced by the database, and the actions performed through application logic, accurately reflect the real policies of the company and can handle real-life situations.
For example, an employee leaving a company might trigger a sequence of actions from the human resources department. The human resources database might also need the flexibility to represent data about a person who has been hired, but not yet started work. Closing an account at an online service might result in data being removed from a database, or the data might be moved or flagged so that it could be recovered if the account is re-opened. A company might establish policies regarding salary maximums, minimums, and adjustments, in addition to basic sanity checks such as the salary not being a negative number. A retail database might not allow a purchase with the same serial number to be returned more than once, or might not allow credit card purchases above a certain value, while a database used to detect fraud might allow these kinds of things.
cache
The general term for any memory area that stores copies of data for frequent or high-speed retrieval. In InnoDB, the primary kind of cache structure is the buffer pool.
cardinality
The number of different values in a table column. When queries refer to columns that have an associated index, the cardinality of each column influences which access method is most efficient. For example, for a column with a unique constraint, the number of different values is equal to the number of rows in the table. If a table has a million rows but only 10 different values for a particular column, each value occurs (on average) 100,000 times. A query such as SELECT c1 FROM t1 WHERE c1 = 50; thus might return 1 row or a huge number of rows, and the database server might process the query differently depending on the cardinality of c1.
If the values in a column have a very uneven distribution, the cardinality might not be a good way to determine the best query plan. For example, SELECT c1 FROM t1 WHERE c1 = x; might return 1 row when x=50 and a million rows when x=30. In such a case, you might need to use index hints to pass along advice about which lookup method is more efficient for a particular query.
Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite index.
For InnoDB, the process of estimating cardinality for indexes is influenced by the innodb_stats_sample_pages and the innodb_stats_on_metadata configuration options. The estimated values are more stable when the persistent statistics feature is enabled (in MySQL 5.6 and higher).
change buffer
A special data structure that records changes to pages in secondary indexes. These values could result from SQL INSERT, UPDATE, or DELETE statements (DML). The set of features involving the change buffer is known collectively as change buffering, consisting of insert buffering, delete buffering, and purge buffering.
Changes are only recorded in the change buffer when the relevant page from the secondary index is not in the buffer pool. When the relevant index page is brought into the buffer pool while associated changes are still in the change buffer, the changes for that page are applied in the buffer pool (merged) using the data from the change buffer. Periodically, the purge operation that runs during times when the system is mostly idle, or during a slow shutdown, writes the new index pages to disk. The purge operation can write the disk blocks for a series of index values more efficiently than if each value were written to disk immediately.
Physically, the change buffer is part of the system tablespace, so that the index changes remain buffered across database restarts. The changes are only applied (merged) when the pages are brought into the buffer pool due to some other read operation.
The kinds and amount of data stored in the change buffer are governed by the innodb_change_buffering and innodb_change_buffer_max_size configuration options. To see information about the current data in the change buffer, issue the SHOW ENGINE INNODB STATUS command.
Formerly known as the insert buffer.
change buffering
The general term for the features involving the change buffer, consisting of insert buffering, delete buffering, and purge buffering. Index changes resulting from SQL statements, which could normally involve random I/O operations, are held back and performed periodically by a background thread. This sequence of operations can write the disk blocks for a series of index values more efficiently than if each value were written to disk immediately. Controlled by the innodb_change_buffering and innodb_change_buffer_max_size configuration options.
checkpoint
As changes are made to data pages that are cached in the buffer pool, those changes are written to the data files sometime later, a process known as flushing. The checkpoint is a record of the latest changes (represented by an LSN value) that have been successfully written to the data files.
checksum
In InnoDB, a validation mechanism to detect corruption when a page in a tablespace is read from disk into the InnoDB buffer pool. This feature is turned on and off by the innodb_checksums configuration option. In MySQL 5.6, you can enable a faster checksum algorithm by also specifying the configuration option innodb_checksum_algorithm=crc32.
The innochecksum command helps to diagnose corruption problems by testing the checksum values for a specified tablespace file while the MySQL server is shut down.
MySQL also uses checksums for replication purposes. For details, see the configuration options binlog_checksum, master_verify_checksum, and slave_sql_verify_checksum.
child table
In a foreign key relationship, a child table is one whose rows refer (or point) to rows in another table with an identical value for a specific column. This is the table that contains the FOREIGN KEY … REFERENCES clause and optionally ON UPDATE and ON DELETE clauses. The corresponding row in the parent table must exist before the row can be created in the child table. The values in the child table can prevent delete or update operations on the parent table, or can cause automatic deletion or updates in the child table, based on the ON CASCADEoption used when creating the foreign key.
clean page
A page in the InnoDB buffer pool where all changes made in memory have also been written (flushed) to the data files. The opposite of a dirty page.
clean shutdown
A shutdown that completes without errors and applies all changes to InnoDB tables before finishing, as opposed to a crash or a fast shutdown. Synonym for slow shutdown.
client
A type of program that sends requests to a server, and interprets or processes the results. The client software might run only some of the time (such as a mail or chat program), and might run interactively (such as the mysql command processor).
client libraries
Files containing collections of functions for working with databases. By compiling your program with these libraries, or installing them on the same system as your application, you can run a database application (known as a client) on a machine that does not have the MySQL server installed; the application accesses the database over a network. With MySQL, you can use the libmysqlclient library from the MySQL server itself.
client-side prepared statement
A type of prepared statement where the caching and reuse are managed locally, emulating the functionality of server-side prepared statements. Historically, used by some Connector/J, Connector/ODBC, and Connector/PHP developers to work around issues with server-side stored procedures. With modern MySQL server versions, server-side prepared statements are recommended for performance, scalability, and memory efficiency.
CLOB
An SQL data type (TINYTEXT, TEXT, MEDIUMTEXT, or LONGTEXT) for objects containing any kind of character data, of arbitrary size. Used for storing text-based documents, with associated character set and collation order. The techniques for handling CLOBs within a MySQL application vary with each Connector and API. MySQL Connector/ODBC defines TEXT values as LONGVARCHAR. For storing binary data, the equivalent is the BLOB type.
clustered index
The InnoDB term for a primary key index. InnoDB table storage is organized based on the values of the primary key columns, to speed up queries and sorts involving the primary key columns. For best performance, choose the primary key columns carefully based on the most performance-critical queries. Because modifying the columns of the clustered index is an expensive operation, choose primary columns that are rarely or never updated.
In the Oracle Database product, this type of table is known as an index-organized table.
cold backup
A backup taken while the database is shut down. For busy applications and web sites, this might not be practical, and you might prefer a warm backup or a hot backup.
column
A data item within a row, whose storage and semantics are defined by a data type. Each table and index is largely defined by the set of columns it contains.
Each column has a cardinality value. A column can be the primary key for its table, or part of the primary key. A column can be subject to a unique constraint, a NOT NULL constraint, or both. Values in different columns, even across different tables, can be linked by a foreign key relationship.
In discussions of MySQL internal operations, sometimes field is used as a synonym.
column index
An index on a single column.
column prefix
When an index is created with a length specification, such as CREATE INDEX idx ON t1 (c1(N)), only the first N characters of the column value are stored in the index. Keeping the index prefix small makes the index compact, and the memory and disk I/O savings help performance. (Although making the index prefix too small can hinder query optimization by making rows with different values appear to the query optimizer to be duplicates.)
For columns containing binary values or long text strings, where sorting is not a major consideration and storing the entire value in the index would waste space, the index automatically uses the first N (typically 768) characters of the value to do lookups and sorts.
command interceptor
Synonym for statement interceptor. One aspect of the interceptor design pattern available for both Connector/NET and Connector/J. What Connector/NET calls a command, Connector/J refers to as a statement. Contrast with exception interceptor.
commit
A SQL statement that ends a transaction, making permanent any changes made by the transaction. It is the opposite of rollback, which undoes any changes made in the transaction.
InnoDB uses an optimistic mechanism for commits, so that changes can be written to the data files before the commit actually occurs. This technique makes the commit itself faster, with the tradeoff that more work is required in case of a rollback.
By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement.
compact row format
The default InnoDB row format since MySQL 5.0.3. Available for tables that use the Antelope file format. It has a more compact representation for nulls and variable-length fields than the prior default (redundant row format).
Because of the B-tree indexes that make row lookups so fast in InnoDB, there is little if any performance benefit to keeping all rows the same size.
For additional information about InnoDB COMPACT row format, see Section 14.9.4, “COMPACT and REDUNDANT Row Formats”.
composite index
An index that includes multiple columns.
compressed backup
The compression feature of the MySQL Enterprise Backup product makes a compressed copy of each tablespace, changing the extension from .ibd to .ibz. Compressing the backup data allows you to keep more backups on hand, and reduces the time to transfer backups to a different server. The data is uncompressed during the restore operation. When a compressed backup operation processes a table that is already compressed, it skips the compression step for that table, because compressing again would result in little or no space savings.
A set of files produced by the MySQL Enterprise Backup product, where each tablespace is compressed. The compressed files are renamed with a .ibz file extension.
Applying compression right at the start of the backup process helps to avoid storage overhead during the compression process, and to avoid network overhead when transferring the backup files to another server. The process of applying the binary log takes longer, and requires uncompressing the backup files.
compressed row format
A row format that enables data and index compression for InnoDB tables. It was introduced in the InnoDB Plugin, available as part of the Barracuda file format. Large fields are stored away from the page that holds the rest of the row data, as in dynamic row format. Both index pages and the large fields are compressed, yielding memory and disk savings. Depending on the structure of the data, the decrease in memory and disk usage might or might not outweigh the performance overhead of uncompressing the data as it is used. See Section 14.7, “InnoDB Compressed Tables” for usage details.
For additional information about InnoDB COMPRESSED row format, see Section 14.9.3, “DYNAMIC and COMPRESSED Row Formats”.
compressed table
A table for which the data is stored in compressed form. For InnoDB, it is a table created with ROW_FORMAT=COMPRESSED. SeeSection 14.7, “InnoDB Compressed Tables” for more information.
compression
A feature with wide-ranging benefits from using less disk space, performing less I/O, and using less memory for caching. InnoDB table and index data can be kept in a compressed format during database operation.
The data is uncompressed when needed for queries, and re-compressed when changes are made by DML operations. After you enable compression for a table, this processing is transparent to users and application developers. DBAs can consult information_schema tables to monitor how efficiently the compression parameters work for the MySQL instance and for particular compressed tables.
When InnoDB table data is compressed, the compression applies to the table itself, any associated index data, and the pages loaded into the buffer pool. Compression does not apply to pages in the undo buffer.
The table compression feature requires using MySQL 5.5 or higher, or the InnoDB Plugin in MySQL 5.1 or earlier, and creating the table using the Barracuda file format and compressed row format, with the innodb_file_per_table setting turned on. The compression for each table is influenced by the KEY_BLOCK_SIZE clause of the CREATE TABLE and ALTER TABLE statements. In MySQL 5.6 and higher, compression is also affected by the server-wide configuration options innodb_compression_failure_threshold_pct, innodb_compression_level, and innodb_compression_pad_pct_max. See Section 14.7, “InnoDB Compressed Tables” for usage details.
Another type of compression is the compressed backup feature of the MySQL Enterprise Backup product.
compression failure
Not actually an error, rather an expensive operation that can occur when using compression in combination with DML operations. It occurs when: updates to a compressed page overflow the area on the page reserved for recording modifications; the page is compressed again, with all changes applied to the table data; the re-compressed data does not fit on the original page, requiring MySQL to split the data into two new pages and compress each one separately. To check the frequency of this condition, query the table INFORMATION_SCHEMA.INNODB_CMPand check how much the value of the COMPRESS_OPS column exceeds the value of the COMPRESS_OPS_OK column. Ideally, compression failures do not occur often; when they do, you can adjust the configuration options innodb_compression_level,innodb_compression_failure_threshold_pct, and innodb_compression_pad_pct_max.
concatenated index
See composite index.
concurrency
The ability of multiple operations (in database terminology, transactions) to run simultaneously, without interfering with each other. Concurrency is also involved with performance, because ideally the protection for multiple simultaneous transactions works with a minimum of performance overhead, using efficient mechanisms for locking.
configuration file
The file that holds the option values used by MySQL at startup. Traditionally, on Linux and UNIX this file is named my.cnf, and on Windows it is named my.ini. You can set a number of options related to InnoDB under the [mysqld] section of the file.
Typically, this file is searched for in the locations /etc/my.cnf /etc/mysql/my.cnf /usr/local/mysql/etc/my.cnf and ~/.my.cnf. See Section 4.2.6, “Using Option Files” for details about the search path for this file.
When you use the MySQL Enterprise Backup product, you typically use two configuration files: one that specifies where the data comes from and how it is structured (which could be the original configuration file for your real server), and a stripped-down one containing only a small set of options that specify where the backup data goes and how it is structured. The configuration files used with the MySQL Enterprise Backup product must contain certain options that are typically left out of regular configuration files, so you might need to add some options to your existing configuration file for use with MySQL Enterprise Backup.
consistent read
A read operation that uses snapshot information to present query results based on a point in time, regardless of changes performed by other transactions running at the same time. If queried data has been changed by another transaction, the original data is reconstructed based on the contents of the undo log. This technique avoids some of the locking issues that can reduce concurrency by forcing transactions to wait for other transactions to finish.
With the repeatable read isolation level, the snapshot is based on the time when the first read operation is performed. With the read committed isolation level, the snapshot is reset to the time of each consistent read operation.
Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels. Because a consistent read does not set any locks on the tables it accesses, other sessions are free to modify those tables while a consistent read is being performed on the table.
For technical details about the applicable isolation levels, see Section 14.2.4, “Consistent Nonlocking Reads”.
constraint
An automatic test that can block database changes to prevent data from becoming inconsistent. (In computer science terms, a kind of assertion related to an invariant condition.) Constraints are a crucial component of the ACID philosophy, to maintain data consistency. Constraints supported by MySQL include FOREIGN KEY constraints and unique constraints.
counter
A value that is incremented by a particular kind of InnoDB operation. Useful for measuring how busy a server is, troubleshooting the sources of performance issues, and testing whether changes (for example, to configuration settings or indexes used by queries) have the desired low-level effects. Different kinds of counters are available through performance_schema tables and information_schema tables, particularlyinformation_schema.innodb_metrics.
covering index
An index that includes all the columns retrieved by a query. Instead of using the index values as pointers to find the full table rows, the query returns values from the index structure, saving disk I/O. InnoDB can apply this optimization technique to more indexes than MyISAM can, because InnoDB secondary indexes also include the primary key columns. InnoDB cannot apply this technique for queries against tables modified by a transaction, until that transaction ends.
Any column index or composite index could act as a covering index, given the right query. Design your indexes and queries to take advantage of this optimization technique wherever possible.
CPU-bound
A type of workload where the primary bottleneck is CPU operations in memory. Typically involves read-intensive operations where the results can all be cached in the buffer pool.
crash
MySQL uses the term “crash” to refer generally to any unexpected shutdown operation where the server cannot do its normal cleanup. For example, a crash could happen due to a hardware fault on the database server machine or storage device; a power failure; a potential data mismatch that causes the MySQL server to halt; a fast shutdown initiated by the DBA; or many other reasons. The robust, automatic crash recovery for InnoDB tables ensures that data is made consistent when the server is restarted, without any extra work for the DBA.
crash recovery
The cleanup activities that occur when MySQL is started again after a crash. For InnoDB tables, changes from incomplete transactions are replayed using data from the redo log. Changes that were committed before the crash, but not yet written into the data files, are reconstructed from the doublewrite buffer. When the database is shut down normally, this type of activity is performed during shutdown by the purge operation.
During normal operation, committed data can be stored in the change buffer for a period of time before being written to the data files. There is always a tradeoff between keeping the data files up-to-date, which introduces performance overhead during normal operation, and buffering the data, which can make shutdown and crash recovery take longer.
CRUD
Acronym for “create, read, update, delete”, a common sequence of operations in database applications. Often denotes a class of applications with relatively simple database usage (basic DDL, DML and query statements in SQL) that can be implemented quickly in any language.
cursor
An internal data structure that is used to represent the result set of a query, or other operation that performs a search using an SQL WHERE clause. It works like an iterator in other high-level languages, producing each value from the result set as requested.
Although usually SQL handles the processing of cursors for you, you might delve into the inner workings when dealing with performance-critical code.
data dictionary
Metadata that keeps track of InnoDB-related objects such as tables, indexes, and table columns. This metadata is physically located in the InnoDB system tablespace. For historical reasons, it overlaps to some degree with information stored in the .frm files.
Because the MySQL Enterprise Backup product always backs up the system tablespace, all backups include the contents of the data dictionary.
data directory
The directory under which each MySQL instance keeps the data files for InnoDB and the directories representing individual databases. Controlled by the datadir configuration option.
data files
The files that physically contain the InnoDB table and index data. There can be a one-to-many relationship between data files and tables, as in the case of the system tablespace, which can hold multiple InnoDB tables as well as the data dictionary. There can also be a one-to-one relationship between data files and tables, as when the file-per-table setting is enabled, causing each newly created table to be stored in a separate tablespace.
data warehouse
A database system or application that primarily runs large queries. The read-only or read-mostly data might be organized in denormalized form for query efficiency. Can benefit from the optimizations for read-only transactions in MySQL 5.6 and higher. Contrast with OLTP.
database
Within the MySQL data directory, each database is represented by a separate directory. The InnoDB system tablespace, which can hold table data from multiple databases within a MySQL instance, is kept in its data files that reside outside the individual database directories. When file-per-table mode is enabled, the .ibd files representing individual InnoDB tables are stored inside the database directories.
For long-time MySQL users, a database is a familiar notion. Users coming from an Oracle Database background will find that the MySQL meaning of a database is closer to what Oracle Database calls a schema.
DCL
Data control language, a set of SQL statements for managing privileges. In MySQL, consists of the GRANT and REVOKE statements. Contrast with DDL and DML.
DDL
Data definition language, a set of SQL statements for manipulating the database itself rather than individual table rows. Includes all forms of the CREATE, ALTER, and DROP statements. Also includes the TRUNCATE statement, because it works differently than a DELETE FROM table_name statement, even though the ultimate effect is similar.
DDL statements automatically commit the current transaction; they cannot be rolled back.
The InnoDB online DDL feature enhances performance for CREATE INDEX, DROP INDEX, and many types of ALTER TABLE operations. See Section 15.12, “InnoDB and Online DDL” for more information. Also, the InnoDB file-per-table setting can affect the behavior of DROP TABLE and TRUNCATE TABLE operations.
Contrast with DML and DCL.
deadlock
A situation where different transactions are unable to proceed, because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither will ever release the locks it holds.
A deadlock can occur when the transactions lock rows in multiple tables (through statements such as UPDATE or SELECT … FOR UPDATE), but in the opposite order. A deadlock can also occur when such statements lock ranges of index records and gaps, with each transaction acquiring some locks but not others due to a timing issue.
To reduce the possibility of deadlocks, use transactions rather than LOCK TABLE statements; keep transactions that insert or update data small enough that they do not stay open for long periods of time; when different transactions update multiple tables or large ranges of rows, use the same order of operations (such as SELECT … FOR UPDATE) in each transaction; create indexes on the columns used in SELECT … FOR UPDATE and UPDATE … WHERE statements. The possibility of deadlocks is not affected by the isolation level, because the isolation level changes the behavior of read operations, while deadlocks occur because of write operations.
If a deadlock does occur, InnoDB detects the condition and rolls back one of the transactions (the victim). Thus, even if your application logic is perfectly correct, you must still handle the case where a transaction must be retried. To see the last deadlock in an InnoDB user transaction, use the command SHOW ENGINE INNODB STATUS. If frequent deadlocks highlight a problem with transaction structure or application error handling, run with the innodb_print_all_deadlocks setting enabled to print information about all deadlocks to the mysqld error log.
For background information on how deadlocks are automatically detected and handled, see Section 14.2.10, “Deadlock Detection and Rollback”. For tips on avoiding and recovering from deadlock conditions, see Section 14.2.11, “How to Cope with Deadlocks”.
deadlock detection
A mechanism that automatically detects when a deadlock occurs, and automatically rolls back one of the transactions involved (the victim).
delete
When InnoDB processes a DELETE statement, the rows are immediately marked for deletion and no longer are returned by queries. The storage is reclaimed sometime later, during the periodic garbage collection known as the purge operation, performed by a separate thread. For removing large quantities of data, related operations with their own performance characteristics are truncate and drop.
delete buffering
The technique of storing index changes due to DELETE operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete operations are a two-step process, this operation buffers the write that normally marks an index record for deletion.) It is one of the types of change buffering; the others are insert buffering and purge buffering.
denormalized
A data storage strategy that duplicates data across different tables, rather than linking the tables with foreign keys and join queries. Typically used in data warehouse applications, where the data is not updated after loading. In such applications, query performance is more important than making it simple to maintain consistent data during updates. Contrast with normalized.
descending index
A type of index available with some database systems, where index storage is optimized to process ORDER BY column DESC clauses. Currently, although MySQL allows the DESC keyword in the CREATE TABLE statement, it does not use any special storage layout for the resulting index.
dirty page
A page in the InnoDB buffer pool that has been updated in memory, where the changes are not yet written (flushed) to the data files. The opposite of a clean page.
disk-bound
A type of workload where the primary bottleneck is disk I/O. (Also known as I/O-bound.) Typically involves frequent writes to disk, or random reads of more data than can fit into the buffer pool.
DML
Data manipulation language, a set of SQL statements for performing insert, update, and delete operations. The SELECT statement is sometimes considered as a DML statement, because the SELECT … FOR UPDATE form is subject to the same considerations for locking as INSERT, UPDATE, and DELETE.
DML statements for an InnoDB table operate in the context of a transaction, so their effects can be committed or rolled back as a single unit.
Contrast with DDL and DCL.
document id
In the InnoDB full-text search feature, a special column in the table containing the FULLTEXT index, to uniquely identify the document associated with each ilist value. Its name is FTS_DOC_ID (uppercase required). The column itself must be of BIGINT UNSIGNED NOT NULL type, with a unique index named FTS_DOC_ID_INDEX. Preferably, you define this column when creating the table. If InnoDB must add the column to the table while creating a FULLTEXT index, the indexing operation is considerably more expensive.
doublewrite buffer
InnoDB uses a novel file flush technique called doublewrite. Before writing pages to the data files, InnoDB first writes them to a contiguous area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer have completed, does InnoDB write the pages to their proper positions in the data file. If there is an operating system, storage subsystem, or mysqld process crash in the middle of a page write, InnoDB can later find a good copy of the page from the doublewrite buffer during crash recovery.
Although data is always written twice, the doublewrite buffer does not require twice as much I/O overhead or twice as many I/O operations. Data is written to the buffer itself as a large sequential chunk, with a single fsync() call to the operating system.
To turn off the doublewrite buffer, specify the option innodb_doublewrite=0.
drop
A kind of DDL operation that removes a schema object, through a statement such as DROP TABLE or DROP INDEX. It maps internally to an ALTER TABLE statement. From an InnoDB perspective, the performance considerations of such operations involve the time that the data dictionary is locked to ensure that interrelated objects are all updated, and the time to update memory structures such as the buffer pool. For atable, the drop operation has somewhat different characteristics than a truncate operation (TRUNCATE TABLE statement).
dynamic row format
A row format introduced in the InnoDB Plugin, available as part of the Barracuda file format. Because TEXT and BLOB fields are stored outside of the rest of the page that holds the row data, it is very efficient for rows that include large objects. Since the large fields are typically not accessed to evaluate query conditions, they are not brought into the buffer pool as often, resulting in fewer I/O operations and better utilization of cache memory.
For additional information about InnoDB DYNAMIC row format, see Section 14.9.3, “DYNAMIC and COMPRESSED Row Formats”.
early adopter
A stage similar to beta, when a software product is typically evaluated for performance, functionality, and compatibility in a non-mission-critical setting. InnoDB uses the early adopter designation rather than beta, through a succession of point releases leading up to a GA release.
error log
A type of log showing information about MySQL startup and critical runtime errors and crash information. For details, see Section 5.2.2, “The Error Log”.
eviction
The process of removing an item from a cache or other temporary storage area, such as the InnoDB buffer pool. Often, but not always, uses theLRU algorithm to determine which item to remove. When a dirty page is evicted, its contents are flushed to disk, and any dirty neighbor pages might be flushed also.
exclusive lock
A kind of lock that prevents any other transaction from locking the same row. Depending on the transaction isolation level, this kind of lock might block other transactions from writing to the same row, or might also block other transactions from reading the same row. The default InnoDB isolation level, REPEATABLE READ, enables higher concurrency by allowing transactions to read rows that have exclusive locks, a technique known as consistent read.
extent
A group of pages within a tablespace totaling 1 megabyte (or 1048576 bytes). With the default page size of 16KB, an extent contains 64 pages. In MySQL 5.6, the page size for an InnoDB instance can be 4KB, 8KB, or 16KB, controlled by the innodb_page_size configuration option. If a 4KB or 8KB page size is used, the extent contains more pages but is still 1MB in size.
InnoDB features such as segments, read-ahead requests and the doublewrite buffer use I/O operations that read, write, allocate, or free data one extent at a time.
fast shutdown
The default shutdown procedure for InnoDB, based on the configuration setting innodb_fast_shutdown=1. To save time, certainflush operations are skipped. This type of shutdown is safe during normal usage, because the flush operations are performed during the next startup, using the same mechanism as in crash recovery. In cases where the database is being shut down for an upgrade or downgrade, do aslow shutdown instead to ensure that all relevant changes are applied to the data files during the shutdown.
file format
The format used by InnoDB for each table, typically with the file-per-table setting enabled so that each table is stored in a separate .ibd file. Currently, the file formats available in InnoDB are known as Antelope and Barracuda. Each file format supports one or more row formats. The row formats available for Barracuda tables, COMPRESSED and DYNAMIC, enable important new storage features for InnoDB tables.
file-per-table
A general name for the setting controlled by the innodb_file_per_table option. That is a very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default.
For each table created while this setting is in effect, the data is stored in a separate .ibd file rather than in the ibdata files of the system tablespace. When table data is stored in individual files, you have more flexibility to choose nondefault file formats and row formats, which are required for features such as data compression. The TRUNCATE TABLE operation is also much faster, and the reclaimed space can be used by the operating system rather than remaining reserved for InnoDB.
The MySQL Enterprise Backup product is more flexible for tables that are in their own files. For example, tables can be excluded from a backup, but only if they are in separate files. Thus, this setting is suitable for tables that are backed up less frequently or on a different schedule.
fill factor
In an InnoDB index, the proportion of a page that is taken up by index data before the page is split. The unused space when index data is first divided between pages allows for rows to be updated with longer string values without requiring expensive index maintenance operations. If the fill factor is too low, the index consumes more space than needed, causing extra I/O overhead when reading the index. If the fill factor is too high, any update that increases the length of column values can cause extra I/O overhead for index maintenance. See Section 14.2.13.4, “Physical Structure of an InnoDB Index” for more information.
fixed row format
This row format is used by the MyISAM storage engine, not by InnoDB. If you create an InnoDB table with the option row_format=fixed, InnoDB translates this option to use the compact row format instead, although the fixed value might still show up in output such as SHOW TABLE STATUS reports.
flush
To write changes to the database files, that had been buffered in a memory area or a temporary disk storage area. The InnoDB storage structures that are periodically flushed include the redo log, the undo log, and the buffer pool.
Flushing can happen because a memory area becomes full and the system needs to free some space, because a commit operation means the changes from a transaction can be finalized, or because a slow shutdown operation means that all outstanding work should be finalized. When it is not critical to flush all the buffered data at once, InnoDB can use a technique called fuzzy checkpointing to flush small batches of pages to spread out the I/O overhead.
flush list
An internal InnoDB data structure that tracks dirty pages in the buffer pool: that is, pages that have been changed and need to be written back out to disk. This data structure is updated frequently by InnoDB’s internal mini-transactions, and so is protected by its own mutex to allow concurrent access to the buffer pool.
foreign key
A type of pointer relationship, between rows in separate InnoDB tables. The foreign key relationship is defined on one column in both the parent table and the child table.
In addition to enabling fast lookup of related information, foreign keys help to enforce referential integrity, by preventing any of these pointers from becoming invalid as data is inserted, updated, and deleted. This enforcement mechanism is a type of constraint. A row that points to another table cannot be inserted if the associated foreign key value does not exist in the other table. If a row is deleted or its foreign key value changed, and rows in another table point to that foreign key value, the foreign key can be set up to prevent the deletion, cause the corresponding column values in the other table to become null, or automatically delete the corresponding rows in the other table.
One of the stages in designing a normalized database is to identify data that is duplicated, separate that data into a new table, and set up a foreign key relationship so that the multiple tables can be queried like a single table, using a join operation.
FOREIGN KEY constraint
The type of constraint that maintains database consistency through a foreign key relationship. Like other kinds of constraints, it can prevent data from being inserted or updated if data would become inconsistent; in this case, the inconsistency being prevented is between data in multiple tables. Alternatively, when a DML operation is performed, FOREIGN KEY constraints can cause data in child rows to be deleted, changed to different values, or set to null, based on the ON CASCADE option specified when creating the foreign key.
FTS
In most contexts, an acronym for full-text search. Sometimes in performance discussions, an acronym for full table scan.
full backup
A backup that includes all the tables in each MySQL database, and all the databases in a MySQL instance. Contrast with partial backup.
full table scan
An operation that requires reading the entire contents of a table, rather than just selected portions using an index. Typically performed either with small lookup tables, or in data warehousing situations with large tables where all available data is aggregated and analyzed. How frequently these operations occur, and the sizes of the tables relative to available memory, have implications for the algorithms used in query optimization and managing the buffer pool.
The purpose of indexes is to allow lookups for specific values or ranges of values within a large table, thus avoiding full table scans when practical.
full-text search
The MySQL feature for finding words, phrases, Boolean combinations of words, and so on within table data, in a faster, more convenient, and more flexible way than using the SQL LIKE operator or writing your own application-level search algorithm. It uses the SQL function MATCH()and FULLTEXT indexes.
FULLTEXT index
The special kind of index that holds the search index in the MySQL full-text search mechanism. Represents the words from values of a column, omitting any that are specified as stopwords. Originally, only available for MyISAM tables. Starting in MySQL 5.6.4, it is also available for InnoDB tables.
fuzzy checkpointing
A technique that flushes small batches of dirty pages from the buffer pool, rather than flushing all dirty pages at once which would disrupt database processing.
GA
“Generally available”, the stage when a software product leaves beta and is available for sale, official support, and production use.
gap
A place in an InnoDB index data structure where new values could be inserted. When you lock a set of rows with a statement such as SELECT … FOR UPDATE, InnoDB can create locks that apply to the gaps as well as the actual values in the index. For example, if you select all values greater than 10 for update, a gap lock prevents another transaction from inserting a new value that is greater than 10. The supremum record and infimum record represent the gaps containing all values greater than or less than all the current index values.
gap lock
A lock on a gap between index records, or a lock on the gap before the first or after the last index record. For example, SELECT c1 FOR UPDATE FROM t WHERE c1 BETWEEN 10 and 20; prevents other transactions from inserting a value of 15 into the column t.c1, whether or not there was already any such value in the column, because the gaps between all existing values in the range are locked. Contrast with record lock and next-key lock.
Gap locks are part of the tradeoff between performance and concurrency, and are used in some transaction isolation levels and not others.
general query log
A type of log used for diagnosis and troubleshooting of SQL statements processed by the MySQL server. Can be stored in a file or in a database table. You must enable this feature through the general_log configuration option to use it. You can disable it for a specific connection through the sql_log_off configuration option.
Records a broader range of queries than the slow query log. Unlike the binary log, which is used for replication, the general query log contains SELECT statements and does not maintain strict ordering. For more information, see Section 5.2.3, “The General Query Log”.
global_transaction
A type of transaction involved in XA operations. It consists of several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends ACID properties “up a level” so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. For this type of distributed transaction, you must use the SERIALIZABLE isolation level to achieve ACID properties.
group commit
An InnoDB optimization that performs some low-level I/O operations (log write) once for a set of commit operations, rather than flushing and syncing separately for each commit.
When the binary log is enabled, you typically also set the configuration option sync_binlog=0, because group commit for the binary log is only supported if it is set to 0.
hash index
A type of index intended for queries that use equality operators, rather than range operators such as greater-than or BETWEEN. It is available for MEMORY tables. Although hash indexes are the default for MEMORY tables for historic reasons, that storage engine also supports B-tree indexes, which are often a better choice for general-purpose queries.
MySQL includes a variant of this index type, the adaptive hash index, that is constructed automatically for InnoDB tables if needed based on runtime conditions.
HDD
Acronym for “hard disk drive”. Refers to storage media using spinning platters, usually when comparing and contrasting with SSD. Its performance characteristics can influence the throughput of a disk-based workload.
heartbeat
A periodic message that is sent to indicate that a system is functioning properly. In a replication context, if the master stops sending such messages, one of the slaves can take its place. Similar techniques can be used between the servers in a cluster environment, to confirm that all of them are operating properly.
high-water mark
A value representing an upper limit, either a hard limit that should not be exceeded at runtime, or a record of the maximum value that was actually reached. Contrast with low-water mark.
history list
A list of transactions with delete-marked records scheduled to be processed by the InnoDB purge operation. Recorded in the undo log. The length of the history list is reported by the command SHOW ENGINE INNODB STATUS. If the history list grows longer than the value of the innodb_max_purge_lag configuration option, each DML operation is delayed slightly to allow the purge operation to finish flushing the deleted records.
Also known as purge lag.
hot
A condition where a row, table, or internal data structure is accessed so frequently, requiring some form of locking or mutual exclusion, that it results in a performance or scalability issue.
Although “hot” typically indicates an undesirable condition, a hot backup is the preferred type of backup.
hot backup
A backup taken while the database and is running and applications are reading and writing to it. The backup involves more than simply copying data files: it must include any data that was inserted or updated while the backup was in process; it must exclude any data that was deleted while the backup was in process; and it must ignore any changes that were not committed.
The Oracle product that performs hot backups, of InnoDB tables especially but also tables from MyISAM and other storage engines, is known as MySQL Enterprise Backup.
The hot backup process consists of two stages. The initial copying of the data files produces a raw backup. The apply step incorporates any changes to the database that happened while the backup was running. Applying the changes produces a prepared backup; these files are ready to be restored whenever necessary.
ib-file set
The set of files managed by InnoDB within a MySQL database: the system tablespace, any file-per-table tablespaces, and the (typically 2)redo log files. Used sometimes in detailed discussions of InnoDB file structures and formats, to avoid ambiguity between the meanings ofdatabase between different DBMS products, and the non-InnoDB files that may be part of a MySQL database.
ibbackup_logfile
A supplemental backup file created by the MySQL Enterprise Backup product during a hot backup operation. It contains information about any data changes that occurred while the backup was running. The initial backup files, including ibbackup_logfile, are known as a raw backup, because the changes that occurred during the backup operation are not yet incorporated. After you perform the apply step to the raw backup files, the resulting files do include those final data changes, and are known as a prepared backup. At this stage, theibbackup_logfile file is no longer necessary.
ibdata file
A set of files with names such as ibdata1, ibdata2, and so on, that make up the InnoDB system tablespace. These files contain metadata about InnoDB tables, (the data dictionary), and the storage areas for the undo log, the change buffer, and the doublewrite buffer. They also can contain some or all of the table data also (depending on whether the file-per-table mode is in effect when each table is created). When the innodb_file_per_table option is enabled, data and indexes for newly created tables are stored in separate .ibd files rather than in the system tablespace.
The growth of the ibdata files is influenced by the innodb_autoextend_increment configuration option.
ibtmp file
The InnoDB temporary tablespace data file for non-compressed InnoDB temporary tables and related objects. The configuration file option,innodb_temp_data_file_path, allows users to define a relative path for the temporary data file. Ifinnodb_temp_data_file_path is not specified, the default behavior is to create a single auto-extending 12MB data file namedibtmp1 in the data directory, alongside ibdata1.
ib_logfile
A set of files, typically named ib_logfile0 and ib_logfile1, that form the redo log. Also sometimes referred to as the log group. These files record statements that attempt to change data in InnoDB tables. These statements are replayed automatically to correct data written by incomplete transactions, on startup following a crash.
This data cannot be used for manual recovery; for that type of operation, use the binary log.
ilist
Within an InnoDB FULLTEXT index, the data structure consisting of a document ID and positional information for a token (that is, a particular word).
implicit row lock
A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.
implicit row lock
A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.
implicit row lock
A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.
in-memory database
A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the “D” in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory.
MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.
in-memory database
A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the “D” in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory.
MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.
in-memory database
A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the “D” in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory.
MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.
incremental backup
A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation.
The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.
incremental backup
A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation.
The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.
incremental backup
A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation.
The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.
index
A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns.
InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes.
Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables.
Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in the MEMORY storage engine and the InnoDB adaptive hash index.
index
A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns.
InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes.
Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables.
Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in the MEMORY storage engine and the InnoDB adaptive hash index.
index
A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns.
InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes.
Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables.
Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in the MEMORY storage engine and the InnoDB adaptive hash index.
index cache
A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written to disk when the index cache becomes full. Each InnoDB FULLTEXTindex has its own separate index cache, whose size is controlled by the configuration option innodb_ft_cache_size.
index cache
A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written to disk when the index cache becomes full. Each InnoDB FULLTEXTindex has its own separate index cache, whose size is controlled by the configuration option innodb_ft_cache_size.
index cache
A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written to disk when the index cache becomes full. Each InnoDB FULLTEXTindex has its own separate index cache, whose size is controlled by the configuration option innodb_ft_cache_size.
index hint
Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE INDEX, USE INDEX, andIGNORE INDEX clauses. Typically used when indexed columns have unevenly distributed values, resulting in inaccurate cardinalityestimates.
index hint
Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE INDEX, USE INDEX, andIGNORE INDEX clauses. Typically used when indexed columns have unevenly distributed values, resulting in inaccurate cardinalityestimates.
index hint
Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE INDEX, USE INDEX, andIGNORE INDEX clauses. Typically used when indexed columns have unevenly distributed values, resulting in inaccurate cardinalityestimates.
index prefix
In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.
index prefix
In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.
index prefix
In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.
infimum record
A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction has a statement such as SELECT … FOR UPDATE … WHERE col < 10;, and the smallest value in the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller values such as 0, -10, and so on.
infimum record
A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction has a statement such as SELECT … FOR UPDATE … WHERE col < 10;, and the smallest value in the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller values such as 0, -10, and so on.
infimum record
A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction has a statement such as SELECT … FOR UPDATE … WHERE col < 10;, and the smallest value in the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller values such as 0, -10, and so on.
INFORMATION_SCHEMA
The name of the database that provides a query interface to the MySQL data dictionary. (This name is defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query tables such as INFORMATION_SCHEMA.TABLES and INFORMATION_SCHEMA.COLUMNS, rather than using SHOW commands that produce unstructured output.
The information schema contains some tables that are specific to InnoDB, such as INNODB_LOCKS and INNODB_TRX. You use these tables not to see how the database is structured, but to get real-time information about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In particular, these tables provide information about MySQL features related to compression, and transactions and their associated locks.
INFORMATION_SCHEMA
The name of the database that provides a query interface to the MySQL data dictionary. (This name is defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query tables such as INFORMATION_SCHEMA.TABLES and INFORMATION_SCHEMA.COLUMNS, rather than using SHOW commands that produce unstructured output.
The information schema contains some tables that are specific to InnoDB, such as INNODB_LOCKS and INNODB_TRX. You use these tables not to see how the database is structured, but to get real-time information about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In particular, these tables provide information about MySQL features related to compression, and transactions and their associated locks.
INFORMATION_SCHEMA
The name of the database that provides a query interface to the MySQL data dictionary. (This name is defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query tables such as INFORMATION_SCHEMA.TABLES and INFORMATION_SCHEMA.COLUMNS, rather than using SHOW commands that produce unstructured output.
The information schema contains some tables that are specific to InnoDB, such as INNODB_LOCKS and INNODB_TRX. You use these tables not to see how the database is structured, but to get real-time information about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In particular, these tables provide information about MySQL features related to compression, and transactions and their associated locks.
InnoDB
A MySQL component that combines high performance with transactional capability for reliability, robustness, and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it handles tables created or altered with the ENGINE=INNODB clause. See Chapter 14, The InnoDB Storage Engine for architectural details and administration procedures, and Section 8.5, “Optimizing for InnoDB Tables” for performance advice.
In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the component known as the InnoDB Plugin. See Section 14.1.1, “InnoDB as the Default MySQL Storage Engine” for the considerations involved in transitioning to recent releases where InnoDB tables are the default.
InnoDB tables are ideally suited for hot backups. See Section 25.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.
InnoDB
A MySQL component that combines high performance with transactional capability for reliability, robustness, and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it handles tables created or altered with the ENGINE=INNODB clause. See Chapter 14, The InnoDB Storage Engine for architectural details and administration procedures, and Section 8.5, “Optimizing for InnoDB Tables” for performance advice.
In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the component known as the InnoDB Plugin. See Section 14.1.1, “InnoDB as the Default MySQL Storage Engine” for the considerations involved in transitioning to recent releases where InnoDB tables are the default.
InnoDB tables are ideally suited for hot backups. See Section 25.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.
InnoDB
A MySQL component that combines high performance with transactional capability for reliability, robustness, and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it handles tables created or altered with the ENGINE=INNODB clause. See Chapter 14, The InnoDB Storage Engine for architectural details and administration procedures, and Section 8.5, “Optimizing for InnoDB Tables” for performance advice.
In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the component known as the InnoDB Plugin. See Section 14.1.1, “InnoDB as the Default MySQL Storage Engine” for the considerations involved in transitioning to recent releases where InnoDB tables are the default.
InnoDB tables are ideally suited for hot backups. See Section 25.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.
innodb_autoinc_lock_mode
The innodb_autoinc_lock_mode option controls the algorithm used for auto-increment locking. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb_autoinc_lock_mode=1. This setting is known as consecutive lock mode, because multi-row inserts within a transaction receive consecutive auto-increment values. If you have innodb_autoinc_lock_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. This setting is known as interleaved lock mode, because multiple multi-row insert statements running at the same time can receive autoincrement values that are interleaved. The setting innodb_autoinc_lock_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.
innodb_autoinc_lock_mode
The innodb_autoinc_lock_mode option controls the algorithm used for auto-increment locking. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb_autoinc_lock_mode=1. This setting is known as consecutive lock mode, because multi-row inserts within a transaction receive consecutive auto-increment values. If you have innodb_autoinc_lock_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. This setting is known as interleaved lock mode, because multiple multi-row insert statements running at the same time can receive autoincrement values that are interleaved. The setting innodb_autoinc_lock_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.
innodb_autoinc_lock_mode
The innodb_autoinc_lock_mode option controls the algorithm used for auto-increment locking. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb_autoinc_lock_mode=1. This setting is known as consecutive lock mode, because multi-row inserts within a transaction receive consecutive auto-increment values. If you have innodb_autoinc_lock_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. This setting is known as interleaved lock mode, because multiple multi-row insert statements running at the same time can receive autoincrement values that are interleaved. The setting innodb_autoinc_lock_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.
innodb_file_format
The innodb_file_format option determines the file format for all InnoDB tablespaces created after you specify a value for this option. To create tablespaces other than the system tablespace, you must also use the file-per-table option. Currently, you can specify the Antelopeand Barracuda file formats.
innodb_file_format
The innodb_file_format option determines the file format for all InnoDB tablespaces created after you specify a value for this option. To create tablespaces other than the system tablespace, you must also use the file-per-table option. Currently, you can specify the Antelopeand Barracuda file formats.
innodb_file_format
The innodb_file_format option determines the file format for all InnoDB tablespaces created after you specify a value for this option. To create tablespaces other than the system tablespace, you must also use the file-per-table option. Currently, you can specify the Antelopeand Barracuda file formats.
innodb_file_per_table
A very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. The innodb_file_per_table option turns on file-per-table mode, which stores each newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace.
This option affects the performance and storage considerations for a number of SQL statements, such as DROP TABLE and TRUNCATE TABLE.
This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup.
This option was once static, but can now be set using the SET GLOBAL command.
For reference information, see innodb_file_per_table. For usage information, see Section 14.5.2, “InnoDB File-Per-Table Mode”.
innodb_file_per_table
A very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. The innodb_file_per_table option turns on file-per-table mode, which stores each newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace.
This option affects the performance and storage considerations for a number of SQL statements, such as DROP TABLE and TRUNCATE TABLE.
This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup.
This option was once static, but can now be set using the SET GLOBAL command.
For reference information, see innodb_file_per_table. For usage information, see Section 14.5.2, “InnoDB File-Per-Table Mode”.
innodb_file_per_table
A very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. The innodb_file_per_table option turns on file-per-table mode, which stores each newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace.
This option affects the performance and storage considerations for a number of SQL statements, such as DROP TABLE and TRUNCATE TABLE.
This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup.
This option was once static, but can now be set using the SET GLOBAL command.
For reference information, see innodb_file_per_table. For usage information, see Section 14.5.2, “InnoDB File-Per-Table Mode”.
innodb_lock_wait_timeout
The innodb_lock_wait_timeout option sets the balance between waiting for shared resources to become available, or giving up and handling the error, retrying, or doing alternative processing in your application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock. Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage engines; such deadlocks are not detected automatically.
innodb_lock_wait_timeout
The innodb_lock_wait_timeout option sets the balance between waiting for shared resources to become available, or giving up and handling the error, retrying, or doing alternative processing in your application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock. Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage engines; such deadlocks are not detected automatically.
innodb_lock_wait_timeout
The innodb_lock_wait_timeout option sets the balance between waiting for shared resources to become available, or giving up and handling the error, retrying, or doing alternative processing in your application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock. Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage engines; such deadlocks are not detected automatically.
innodb_strict_mode
The innodb_strict_mode option controls whether InnoDB operates in strict mode, where conditions that are normally treated as warnings, cause errors instead (and the underlying statements fail).
This mode is the default setting in MySQL 5.5.5 and higher.
innodb_strict_mode
The innodb_strict_mode option controls whether InnoDB operates in strict mode, where conditions that are normally treated as warnings, cause errors instead (and the underlying statements fail).
This mode is the default setting in MySQL 5.5.5 and higher.
innodb_strict_mode
The innodb_strict_mode option controls whether InnoDB operates in strict mode, where conditions that are normally treated as warnings, cause errors instead (and the underlying statements fail).
This mode is the default setting in MySQL 5.5.5 and higher.
insert
One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.
insert
One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.
insert
One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.
insert buffer
Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, “change buffer” is the preferred term.
insert buffer
Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, “change buffer” is the preferred term.
insert buffer
Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, “change buffer” is the preferred term.
insert buffering
The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the types of change buffering; the others are delete buffering and purge buffering.
Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.
insert buffering
The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the types of change buffering; the others are delete buffering and purge buffering.
Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.
insert buffering
The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the types of change buffering; the others are delete buffering and purge buffering.
Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.
instance
A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.
instance
A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.
instance
A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.
instrumentation
Modifications at the source code level to collect performance data for tuning and debugging. In MySQL, data collected by instrumentation is exposed through a SQL interface using the INFORMATION_SCHEMA and PERFORMANCE_SCHEMA databases.
instrumentation
Modifications at the source code level to collect performance data for tuning and debugging. In MySQL, data collected by instrumentation is exposed through a SQL interface using the INFORMATION_SCHEMA and PERFORMANCE_SCHEMA databases.
instrumentation
Modifications at the source code level to collect performance data for tuning and debugging. In MySQL, data collected by instrumentation is exposed through a SQL interface using the INFORMATION_SCHEMA and PERFORMANCE_SCHEMA databases.
intention lock
A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible. For more details on this locking mechanism, seeSection 14.2.3, “InnoDB Lock Modes”.
intention lock
A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible. For more details on this locking mechanism, seeSection 14.2.3, “InnoDB Lock Modes”.
intention lock
A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible. For more details on this locking mechanism, seeSection 14.2.3, “InnoDB Lock Modes”.
inverted index
A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.
inverted index
A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.
inverted index
A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.
IOPS
Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP applications. If this value is near the maximum that the storage devices can handle, the application can become disk-bound, limiting scalability.
IOPS
Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP applications. If this value is near the maximum that the storage devices can handle, the application can become disk-bound, limiting scalability.
IOPS
Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP applications. If this value is near the maximum that the storage devices can handle, the application can become disk-bound, limiting scalability.
isolation level
One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.
From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED.
With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.
isolation level
One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.
From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED.
With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.
isolation level
One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.
From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED.
With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.
join
A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.
join
A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.
join
A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.
KEY_BLOCK_SIZE
An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.
KEY_BLOCK_SIZE
An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.
KEY_BLOCK_SIZE
An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.
latch
A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.
latch
A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.
latch
A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.
list
The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.
list
The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.
list
The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.
lock
The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.
lock
The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.
lock
The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.
lock escalation
An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.
lock escalation
An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.
lock escalation
An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.
lock mode
A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time.
An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time.
Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.
lock mode
A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time.
An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time.
Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.
lock mode
A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time.
An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time.
Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.
locking
The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for goodconcurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.
locking
The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for goodconcurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.
locking
The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for goodconcurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.
locking read
A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT … FOR UPDATE or SELECT … LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a read-only transaction.
locking read
A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT … FOR UPDATE or SELECT … LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a read-only transaction.
locking read
A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT … FOR UPDATE or SELECT … LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a read-only transaction.
log
In the InnoDB context, “log” or “log files” typically refers to the redo log represented by the ib_logfile* files. Another log area, which is physically part of the system tablespace, is the undo log.
Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).
log
In the InnoDB context, “log” or “log files” typically refers to the redo log represented by the ib_logfile* files. Another log area, which is physically part of the system tablespace, is the undo log.
Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).
log
In the InnoDB context, “log” or “log files” typically refers to the redo log represented by the ib_logfile* files. Another log area, which is physically part of the system tablespace, is the undo log.
Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).
log buffer
The memory area that holds data to be written to the log files that make up the redo log. It is controlled by theinnodb_log_buffer_size configuration option.
log buffer
The memory area that holds data to be written to the log files that make up the redo log. It is controlled by theinnodb_log_buffer_size configuration option.
log buffer
The memory area that holds data to be written to the log files that make up the redo log. It is controlled by theinnodb_log_buffer_size configuration option.
log file
One of the ib_logfileN files that make up the redo log. Data is written to these files from the log buffer memory area.
log file
One of the ib_logfileN files that make up the redo log. Data is written to these files from the log buffer memory area.
log file
One of the ib_logfileN files that make up the redo log. Data is written to these files from the log buffer memory area.
log group
The set of files that make up the redo log, typically named ib_logfile0 and ib_logfile1. (For that reason, sometimes referred to collectively as ib_logfile.)
log group
The set of files that make up the redo log, typically named ib_logfile0 and ib_logfile1. (For that reason, sometimes referred to collectively as ib_logfile.)
log group
The set of files that make up the redo log, typically named ib_logfile0 and ib_logfile1. (For that reason, sometimes referred to collectively as ib_logfile.)
logical
A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.
logical
A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.
logical
A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.
logical backup
A backup that reproduces table structure and data, without copying the actual data files. For example, the mysqldump command produces a logical backup, because its output contains statements such as CREATE TABLE and INSERT that can re-create the data. Contrast withphysical backup. A logical backup offers flexibility (for example, you could edit table definitions or insert statements before restoring), but can take substantially longer to restore than a physical backup.
logical backup
A backup that reproduces table structure and data, without copying the actual data files. For example, the mysqldump command produces a logical backup, because its output contains statements such as CREATE TABLE and INSERT that can re-create the data. Contrast withphysical backup. A logical backup offers flexibility (for example, you could edit table definitions or insert statements before restoring), but can take substantially longer to restore than a physical backup.
logical backup
A backup that reproduces table structure and data, without copying the actual data files. For example, the mysqldump command produces a logical backup, because its output contains statements such as CREATE TABLE and INSERT that can re-create the data. Contrast withphysical backup. A logical backup offers flexibility (for example, you could edit table definitions or insert statements before restoring), but can take substantially longer to restore than a physical backup.
loose_
In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.
loose_
In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.
loose_
In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.
low-water mark
A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast withhigh-water mark.
low-water mark
A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast withhigh-water mark.
low-water mark
A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast withhigh-water mark.
LRU
An acronym for “least recently used”, a common method for managing storage areas. The items that have not been used recently are evictedwhen space is needed to cache newer items. InnoDB uses the LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases where a page might be read only a single time, such as during a full table scan. This variation of the LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management differs from the traditional LRU algorithm is fine-tuned by the options innodb_old_blocks_pct, innodb_old_blocks_time, and the new MySQL 5.6 optionsinnodb_lru_scan_depth and innodb_flush_neighbors.
LRU
An acronym for “least recently used”, a common method for managing storage areas. The items that have not been used recently are evictedwhen space is needed to cache newer items. InnoDB uses the LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases where a page might be read only a single time, such as during a full table scan. This variation of the LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management differs from the traditional LRU algorithm is fine-tuned by the options innodb_old_blocks_pct, innodb_old_blocks_time, and the new MySQL 5.6 optionsinnodb_lru_scan_depth and innodb_flush_neighbors.
LRU
An acronym for “least recently used”, a common method for managing storage areas. The items that have not been used recently are evictedwhen space is needed to cache newer items. InnoDB uses the LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases where a page might be read only a single time, such as during a full table scan. This variation of the LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management differs from the traditional LRU algorithm is fine-tuned by the options innodb_old_blocks_pct, innodb_old_blocks_time, and the new MySQL 5.6 optionsinnodb_lru_scan_depth and innodb_flush_neighbors.
LSN
Acronym for “log sequence number”. This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool.
Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values.
In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup.
LSN
Acronym for “log sequence number”. This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool.
Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values.
In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup.
LSN
Acronym for “log sequence number”. This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool.
Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values.
In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup.
master server
Frequently shortened to “master”. A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.
master server
Frequently shortened to “master”. A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.
master server
Frequently shortened to “master”. A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.
master thread
An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes.
To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.
master thread
An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes.
To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.
master thread
An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes.
To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.
memcached
A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching.
The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.
memcached
A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching.
The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.
memcached
A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching.
The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.
merge
To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in thechange buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flushmechanism.
merge
To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in thechange buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flushmechanism.
merge
To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in thechange buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flushmechanism.
metadata lock
A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking”.
Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the amount of metadata locking. The objective is for DDL operations that do not change the table structure (such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried, updated, and so on by other transactions.
metadata lock
A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking”.
Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the amount of metadata locking. The objective is for DDL operations that do not change the table structure (such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried, updated, and so on by other transactions.
metadata lock
A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking”.
Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the amount of metadata locking. The objective is for DDL operations that do not change the table structure (such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried, updated, and so on by other transactions.
metrics counter
A feature implemented by the innodb_metrics table in the information_schema, in MySQL 5.6 and higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance tuning in combination with data from the performance_schema.
metrics counter
A feature implemented by the innodb_metrics table in the information_schema, in MySQL 5.6 and higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance tuning in combination with data from the performance_schema.
metrics counter
A feature implemented by the innodb_metrics table in the information_schema, in MySQL 5.6 and higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance tuning in combination with data from the performance_schema.
midpoint insertion strategy
The technique of initially bringing pages into the InnoDB buffer pool not at the “newest” end of the list, but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the innodb_old_blocks_pct option. The intent is that blocks that are only read once, such as during a full table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.
midpoint insertion strategy
The technique of initially bringing pages into the InnoDB buffer pool not at the “newest” end of the list, but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the innodb_old_blocks_pct option. The intent is that blocks that are only read once, such as during a full table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.
midpoint insertion strategy
The technique of initially bringing pages into the InnoDB buffer pool not at the “newest” end of the list, but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the innodb_old_blocks_pct option. The intent is that blocks that are only read once, such as during a full table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.
mini-transaction
An internal phase of InnoDB processing, when making changes at the physical level to internal data structures during DML operations. A mini-transaction (mtr) has no notion of rollback; multiple mini-transactions can occur within a single transaction. Mini-transactions write information to the redo log that is used during crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example during purge processing by background threads.
mini-transaction
An internal phase of InnoDB processing, when making changes at the physical level to internal data structures during DML operations. A mini-transaction (mtr) has no notion of rollback; multiple mini-transactions can occur within a single transaction. Mini-transactions write information to the redo log that is used during crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example during purge processing by background threads.
mini-transaction
An internal phase of InnoDB processing, when making changes at the physical level to internal data structures during DML operations. A mini-transaction (mtr) has no notion of rollback; multiple mini-transactions can occur within a single transaction. Mini-transactions write information to the redo log that is used during crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example during purge processing by background threads.
