| mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.b1= t2.b2 AND t1.a1=10); |
| +----+--------------------+-------+------+------+-------------+ | id | select_type | table | type | key | Extra | +----+--------------------+-------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | Using where | | 2 | DEPENDENT SUBQUERY | t2 | ALL | NULL | Using where | +----+--------------------+-------+------+------+-------------+ 2 rows in set, 3 warnings (0.02 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 exists(/* select#2 */ select 1 from `test`.`t2` where ((`test`.`t1`.`b1` = `test`.`t2`.`b2`) and (`test`.`t1`.`a1` = 10)) ) |
从查询执行计划看，子查询存在，MySQL没有进一步做子查询的优化工作。
示例二，MySQL不支持对NOT EXISTS类型的子查询的优化：
NOT EXISTS类型的相关子查询，查询执行计划如下：
| mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t1.a1= t2.a2 AND t2.a2>10); |
| +----+--------------------+-------+------+------+-------------+ | id | select_type | table | type | key | Extra | +----+--------------------+-------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | Using where | | 2 | DEPENDENT SUBQUERY | t2 | ALL | NULL | Using where | +----+--------------------+-------+------+------+-------------+ 2 rows in set, 2 warnings (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(exists( /* select#2 */ select 1 from `test`.`t2` where ((`test`.`t1`.`a1` = `test`.`t2`.`a2`) and (`test`.`t2`.`a2` > 10)))) ) |
从查询执行计划看，子查询存在，MySQL没有进一步做子查询的优化工作。
另外的一个NOT EXISTS类型的相关子查询，查询执行计划如下：
| mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t1.b1= t2.b2 AND t1.a1=10); |
| +----+--------------------+-------+------+------+-------------+ | id | select_type | table | type | key | Extra | +----+--------------------+-------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | Using where | | 2 | DEPENDENT SUBQUERY | t2 | ALL | NULL | Using where | +----+--------------------+-------+------+------+-------------+ 2 rows in set, 3 warnings (0.00 sec) |
被查询优化器处理后的语句为：
