SQL Optimization

 Query optimization is the process of choosing the most efficient means of executing a SQL statement.

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The database optimizes queries based on statistics collected about the actual data being accessed. The optimizer uses the number of rows, the size of the data set, and other factors to generate possible execution plans, assigning a numeric cost to each plan. The database uses the plan with the lowest cost.

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The database must perform a hard parse at least once for every unique DML statement and performs optimization during this parse. DDL is never optimized unless it includes a DML component such as a subquery that requires optimization.

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See also

• "Overview of the Optimizer"

• Oracle Database SQL Tuning Guide for detailed information about the query optimizer