MySQL 中字符串字段，在使用 in 时，没有加引号时的性能陷阱

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。

原文地址：moell.cn/article/33