MySQL执行计划显示与执行过程不符合 - 数据库编程

--------+---------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 2 | DERIVED | t1 | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 2 | DERIVED | t3 | const | id3 | id3 | 5 | const | 1 | 100.00 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (49.53 sec)

mysql> show warnings;
+-------+------+-----------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-----------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select '5' AS `id1`,'5' AS `a1`,NULL AS `b1` from dual |
+-------+------+-----------------------------------------------------------------------+
1 row in set (0.00 sec)

"标题三"中和"标题一"中的SQL差别在于嵌套的子查询的形式是等号还是IN。

而IN的形式，被MySQL识别优化了标识子查询未“DERIVED ”了，这是MySQL从形式上目前只支持IN形式的子查询优化的优化，对于等号这样的形式，不支持。但是，从等式性质上，最终对"标题一"中的SQL进行了优化。

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MySQL执行计划显示与执行过程不符合(二)