File Organization

Question 1

What are the three levels of abstraction in a DBMS

Answer 1

Conceptual Model eg ERD
Logical Model eg Schema tables
Physical Model eg file organization structure

Question 2

Describe records

Answer 2

Data is usually stored in the form of records.
* Each record consists of a collection of related data values or
items, where each value is formed of one or more bytes and
corresponds to a particular field of the record.
* For example, an EMPLOYEE record would have Name,
Birth_date, Salary, or Supervisor.

Question 3

What is a record type

Answer 3

A collection of field names and their corresponding data
types constitutes a record type or record format definition

Question 4

What are the two types of records

Answer 4

-Fixed length record
-Variable length record

Question 5

Define fixed-length records.

Answer 5

If every record in the file has exactly the same size (in bytes), the
file is said to be made up of fixed-length records.

Question 6

What are some characteristics of fixed length records

Answer 6

1.All the records in the file are of same size.
2. Leads to memory wastage.
3. Access of the records is easier and faster.
4. Exact location of the records can be determined: location of ith record would be.n*(i-1), where n is the size of every record.

Question 7

Define variable-length records

Answer 7

If different records in the file have different sizes, the file is said to be made up of variable-length records. We use Separator characters such as(? Or%) to
determine the number of bytes within a particular
record

Question 8

Give reasons why A file may have variable-length records

Answer 8

• The file records are of the same record type, but one or more of the fields are of varying size (variable-length fields). For example, the Name field of
  EMPLOYEE can be a variable-length field.
• The file records are of the same record type, but one or more of the fields have a repeating field
• The file records are of the same record type, but one or more of the fields are optional; that is, they may have values for some but not all of the file records(optional fields).

Question 9

What is blocking

Answer 9

Blocking: refers to storing a number of records in one block on
the disk

Question 10

What is blocking factor

Answer 10

Blocking factor (bfr) refers to the number of records per block.
* There may be empty space in a block if an integral number of
records do not fit in one block.
*= the size of block/the size of record
*Must be an integer- round down
*Extra space can be used to store metadata (pointers info etc)
* Only applies to fixed length records

Question 11

What are spanned records

Answer 11

Spanned Records: refer to records that exceed the size of one or more blocks and hence span a number of blocks.

Question 12

What is a file description

Answer 12

A file descriptor (or file header ) includes information that
describes the file, such as the field names and their data types,
and the addresses of the file blocks on disk

Question 13

Describe spanned and unspanned blocking and where they are used

Answer 13

File records can be unspanned (no record can span two blocks) or
spanned (a record can be stored in more than one block).
* In a file of fixed-length records, all records have the same format.
Usually, unspanned blocking is used with such files.
* Files of variable-length records require additional information to be stored in each record, such as separator characters and field types. Usually spanned blocking is used with such files.

Question 14

Describe the different ways physical blocks are stored

Answer 14

The physical disk blocks that are allocated to hold the records of a file can be contiguous (first record of the block is indexed), linked, or indexed (every record is indexed).

Question 15

What are the shortcomings of contiguous and indexed storage

Answer 15

Contiguous
-Slower in search
Index
-Extra space for storing indexes

Question 16

How do you utilize unused space in a file with fixed length records

Answer 16

Utilizing unused space in a file with fixed-length records can be done through a process called “record packing” or “record compaction.” The goal is to efficiently manage the file by filling the unused space with additional records, thereby reducing wasted space and optimizing the use of storage.

Question 17

Describe blocking factor in variable length record storage

Answer 17

For variable-length records using
spanned organization, each block may store a different number of records. In this case, the blocking factor bfr represents the average number of records per block for the file

Question 18

How do you calculate the number of blocks needed in variable length storage

Answer 18

record size/ bfr then round UP