| +----+-------------+-------+------+------+-------------+ | id | select_type | table | type | key | Extra | +----+-------------+-------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | Using where | | 2 | SUBQUERY | t2 | ALL | NULL | Using where | +----+-------------+-------+------+------+-------------+ 2 rows in set, 1 warning (0.00 sec) |
被查询优化器处理后的语句为：
| /* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1` from `test`.`t1` where (not(<in_optimizer>( `test`.`t1`.`a1`,`test`.`t1`.`a1` in ( <materialize> (/* select#2 */ select `test`.`t2`.`a2` from `test`.`t2` where (`test`.`t2`.`a2` = 10) having 1 ), <primary_index_lookup>( `test`.`t1`.`a1` in <temporary table> on <auto_key> where ((`test`.`t1`.`a1` = `materialized-subquery`.`a2`)) ) ) )) ) |
从查询执行计划看，表t2做了子查询（SUBQUERY）。而子查询被物化（materialize）。所以，MySQL对于NOT IN子查询采用了物化的优化方式，但不支持子查询的消除。
示例五，MySQL支持对ALL类型的子查询的优化：
不相关的ALL子查询，查询执行计划如下：
| mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >ALL (SELECT a2 FROM t2 WHERE t2.a2>10); |
| +----+-------------+-------+------+------+-------------+ | id | select_type | table | type | key | Extra | +----+-------------+-------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | Using where | | 2 | SUBQUERY | t2 | ALL | NULL | Using where | +----+-------------+-------+------+------+-------------+ 2 rows in set, 1 warning (0.00 sec) |
被查询优化器处理后的语句为：
| /* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1` from `test`.`t1` where <not>((`test`.`t1`.`a1` <= <max>( /* select#2 */ select `test`.`t2`.`a2` from `test`.`t2` where (`test`.`t2`.`a2` > 10) ) )) |
从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“<= <max>”操作符限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>ALL”式的子查询优化，子查询只被执行一次即可求得最大值。
不相关的ALL子查询，查询执行计划如下：
| mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =ALL (SELECT a2 FROM t2 WHERE t2.a2=10); |
