8.2.2 Optimizing Subqueries, Derived Tables, and View References

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

For IN (or =ANY) subqueries, the optimizer has these choices:
- Semijoin
- Materialization
- EXISTS strategy
For NOT IN (or <>ALL) subqueries, the optimizer has these choices:
- Materialization
- EXISTS strategy

For derived tables, the optimizer has these choices (which also apply to view references):

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 and DELETE 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-table UPDATE and DELETE statements that use a join rather than a subquery.