MySQL子查询优化实例

优化：子查询改写成关联查询

线上遇到问题，查询较慢，如为对应SQL的查询执行计划：

localhost.user>explain EXTENDED select  uid from bar_user_6 where bid='' and uid in (SELECT a_uid FROM `user_atten_3` WHERE uid='') limit 12\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: bar_user_6
         type: ref
possible_keys: bid
          key: bid
      key_len: 4
          ref: const
         rows: 581295
     filtered: 100.00
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: user_atten_3
         type: eq_ref
possible_keys: uid
          key: uid
      key_len: 30
          ref: const,func
         rows: 1
     filtered: 100.00
        Extra: Using where; Using index
2 rows in set, 1 warning (0.00 sec)

优化方案，改写成关联查询

localhost.user>explain extended select bar.uid from bar_user_6 as bar ,user_atten_3 as user where bid=''  and bar.uid = user.a_uid and user.uid='' limit 12\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: user
         type: ref
possible_keys: uid
          key: uid
      key_len: 22
          ref: const
         rows: 1
     filtered: 100.00
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: bar
         type: eq_ref
possible_keys: bid
          key: bid
      key_len: 12
          ref: const,user.user.a_uid
         rows: 1
     filtered: 100.00
        Extra: Using index

子查询的查询过程（循环嵌套）：

for each row in t1 matching range {
  for each row in t2 matching reference key {
    for each row in t3 {
      if row satisfies join conditions,
      send to client
    }
  }
}

参考文档：

http://dev.mysql.com/doc/refman/5.5/en/nested-loop-joins.html

http://hidba.org/?p=819

http://hidba.org/?p=624

http://hidba.org/?p=743