场景和环境
redhat6.5 + 64位 + 12核心 + 16g
表数量 600w
mysql 5.0
问题描述
在使用in过程中，同事写了一个简单的in条件查询(字段是普通索引，varchar)，由于拼装sql的时候，没有使用引号，导致出现大量慢查询
问题SQL
select count(*) total from member_phone where phone in(1521xxx541,15845xxx412)
问题SQL和纠正过的写法对比
执行时间
mysql> select count(*) total from member_phone where phone in(1521xxx541,15845xxx412); +-------+ | total | +-------+ | 1 | +-------+ 1 row in set (2.76 sec) mysql> select count(*) total from member_phone where phone in('1521xxx541','15845xxx412'); +-------+ | total | +-------+ | 1 | +-------+ 1 row in set (0.01 sec) mysql> select count(*) total from member_phone where (phone='1521xxx541' or phone='15845xxx412'); +-------+ | total | +-------+ | 1 | +-------+ 1 row in set (0.00 sec)
EXPLAIN
mysql> explain select count(*) total from member_phone where phone in(1521xxx541,15845xxx412) G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: member_phone type: index possible_keys: phone key: phone key_len: 18 ref: NULL rows: 6307075 Extra: Using where; Using index 1 row in set (0.00 sec) mysql> explain select count(*) total from member_phone where phone in('1521xxx541','15845xxx412') G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: member_phone type: range possible_keys: phone key: phone key_len: 18 ref: NULL rows: 2 Extra: Using where; Using index 1 row in set (0.00 sec) mysql> explain select count(*) total from member_phone where (phone='1521xxx541' or phone='15845xxx412') G; *************************** 1. row *************************** id: 1 select_type: SIMPLE table: member_phone type: range possible_keys: phone key: phone key_len: 18 ref: NULL rows: 2 Extra: Using where; Using index 1 row in set (0.01 sec)
总结
在三个类型的sql中，效率从高到低分别是 or,in 添加了引号, in不加引号。在explain中看到不加引号时,显示的用上了索引phone，type 变成了 index ,和全表扫描差不多了，只不过MySQL扫描时按索引的次序进行而不是行。
提醒
在where多个or,in中条件个数比较多,或者多个in 条件时，实际性能都比较差的。以上测试我个人仅在MySQL5.0中测试，高版本官方不知是否优化过。
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