Query Optimization & Conclusion

Question 1

What are the steps in query processing

Answer 1

1.Query
2. Parsor and translator
3. Relational algebra expression
4. Optimiser
5. Execution plan
6. Evaluation engine
7. Query output

Question 2

What are evaluation plans?

Answer 2

For the same sql query, there are many ways to achieve the same result. Each way is called an evaluation plan

Question 3

What is query optimisation?

Answer 3

Choosing the evaluation plan with least cost among all equivalent evaluation plans

Question 4

What could be thought of as cost to a query ?

Answer 4

Cost is generally measured as total time elapsed for answering query
Factors of time cost : Disk accesses, CPU or even network communication .
Typically , predominant cost is disk access.

Question 5

How is disk access cost measured?

Answer 5

Number of seems *average week cost
Number of blocks read *average block-read cost
Number of block written *average block write cost

Question 6

Which cost is bigger ? Cost to read or cost to write disk ?

Answer 6

Cost to write is bigger as after writing disk .. contents have to be read back for confirmation