10.2.2 Optimizing Subqueries, Derived Tables, View References, and Common Table Expressions (original) (raw)

10.2.2 Optimizing Subqueries, Derived Tables, View References, and Common Table Expressions

The MySQL query optimizer has different strategies available to evaluate subqueries:

• For a subquery used with an IN,= ANY, or EXISTS predicate, the optimizer has these choices:
  • Semijoin
  • Materialization
  • EXISTS strategy
• For a subquery used with a NOT IN,<> ALL or NOT EXISTS predicate, the optimizer has these choices:
  • Materialization
  • EXISTS strategy

For a derived table, the optimizer has these choices (which also apply to view references and common table expressions):

• Merge the derived table into the outer query block
• Materialize the derived table to an internal temporary table

The following discussion provides more information about the preceding optimization strategies.

Note

A limitation on UPDATE andDELETE statements that use a subquery to modify a single table is that the optimizer does not use semijoin or materialization subquery optimizations. As a workaround, try rewriting them as multiple-tableUPDATE andDELETE statements that use a join rather than a subquery.