MySQL 中字符串字段,在使用in时,没有加引号时的性能陷阱

2016-12-08 10:12:49   数据库

  mysql  

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:http://moell.cn/article/33