Mysql的一条SQL优化（二）

开发过来查看我做的调整，发现我创建的索引其实在另一个测试库上（测试库2）已经创建好了，但为什么测试库2上还是慢呢？

于是上测试库2查看，发现之前我创建的2个列的索引确实已经有了，并且还多了一个索引i_msource_type：

Indexes Columns
Index_Type

PRIMARY ext_id
Unique

i_mobile mobile

i_msource_type msource_type

i_msource_id msource_id

而msource_type列的可选择性是非常低了，基本上到这里我大概知道是怎么回事了，剩下的事情就是验证我的猜测，explain:

mysql> explain select ext_id, mid, msource_id, msource_type, referee, mobile, tel, mpassword,status from m_vip_ext where (msource_id='xx1391434680574433' and msource_type=1 ) or ( mobile='1391434680574433' and msource_type=1);

+----+-------------+-----------+------+--------------------------------------+----------------+---------+-------+-------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-----------+------+--------------------------------------+----------------+---------+-------+-------+-------------+

| 1 | SIMPLE | m_vip_ext | ref | i_mobile,i_msource_type,i_msource_id | i_msource_type | 4 | const | 58383 | Using where |

+----+-------------+-----------+------+--------------------------------------+----------------+---------+-------+-------+-------------+

1 row in set (0.17 sec)

果不其然，Mysql的优化器并没有选择之前的2个索引，而是选择了可选择性很低的i_msource_type，这无疑是比全表扫描效率更低的一种方式。

果断drop掉索引：

mysql> alter table m_vip_ext drop index i_msource_type;

Query OK, 0 rows affected (0.02 sec)

Records: 0 Duplicates: 0 Warnings: 0

再次explain:

mysql> explain select ext_id, mid, msource_id, msource_type, referee, mobile, tel, mpassword,status from m_vip_ext where (msource_id='xx1391434680574433' and msource_type=1 ) or ( mobile='1391434680574433' and msource_type=1);

+----+-------------+-----------+-------------+-----------------------+-----------------------+---------+------+------+------------------------------------------------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-----------+-------------+-----------------------+-----------------------+---------+------+------+------------------------------------------------------+

| 1 | SIMPLE | m_vip_ext | index_merge | i_mobile,i_msource_id | i_msource_id,i_mobile | 98,99 | NULL | 2 | Using sort_union(i_msource_id,i_mobile); Using where |

+----+-------------+-----------+-------------+-----------------------+-----------------------+---------+------+------+------------------------------------------------------+

1 row in set (0.17 sec)

mysql> explain select ext_id, mid, msource_id, msource_type, referee, mobile, tel, mpassword,status from m_vip_ext where (msource_id='xx1391434680574433' and msource_type=1 ) or ( mobile='1391434680574433' and msource_type=1) \G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: m_vip_ext

type: index_merge

possible_keys: i_mobile,i_msource_id

key: i_msource_id,i_mobile

key_len: 98,99

ref: NULL

rows: 2

Extra: Using sort_union(i_msource_id,i_mobile); Using where

1 row in set (0.00 sec)

ERROR:

No query specified

mysql> show status like '%cost%';

+-----------------+----------+

| Variable_name | Value |

+-----------------+----------+

| Last_query_cost | 7.328006 |

+-----------------+----------+

1 row in set (0.00 sec)

OK了，开发人员用测试库2做的压测，反映并发也能上1000了。