Question

short note on query optimization​

Answer 1

Query optimization is the overall process of choosing the most efficient means of executing a SQL statement. SQL is a nonprocedural language, so the optimizer is free to merge, reorganize, and process in any order. The database optimizes each SQL statement based on statistics collected about the accessed data.


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Answer 2

Answer:

Optimizer

The query optimizer (called simply the optimizer) is built-in database software that determines the most efficient method for a SQL statement to access requested data.

This section contains the following topics:

Purpose of the Query Optimizer

Cost-Based Optimization

Execution Plans

Purpose of the Query Optimizer

The optimizer attempts to generate the most optimal execution plan for a SQL statement.

The optimizer choose the plan with the lowest cost among all considered candidate plans. The optimizer uses available statistics to calculate cost. For a specific query in a given environment, the cost computation accounts for factors of query execution such as I/O, CPU, and communication.

For example, a query might request information about employees who are managers. If the optimizer statistics indicate that 80% of employees are managers, then the optimizer may decide that a full table scan is most efficient. However, if statistics indicate that very few employees are managers, then reading an index followed by a table access by rowid may be more efficient than a full table scan.

Because the database has many internal statistics and tools at its disposal, the optimizer is usually in a better position than the user to determine the optimal method of statement execution. For this reason, all SQL statements use the optimizer.