本文探讨了MySQL在执行SQL之前进行的优化过程，包括如何通过改写查询来提高执行效率。通过示例展示了MySQL如何将计数查询优化，并提供了解释计划和警告信息以帮助理解这一过程。
sql真正被执行前优化其做了哪些sql改写
mysql> explain extended select count(*) from xuehao;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+-------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select count(0) AS `count(*)` from `lina`.`xuehao` |
+-------+------+-------------------------------------------------------------------+
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