MySQL5.7中select半连接优化的示例解析
发布时间：2022-01-10 16:09:58 所属栏目：MySql教程 来源：互联网
导读：这篇文章主要为大家展示了MySQL5.7中select半连接优化的示例分析，内容简而易懂，条理清晰，希望能够帮助大家解决疑惑，下面让小编带领大家一起研究并学习一下MySQL5.7中select半连接优化的示例分析这篇文章吧。 MySQL的子查询一直以来都是性能差的著称，解
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这篇文章主要为大家展示了“MySQL5.7中select半连接优化的示例分析”，内容简而易懂，条理清晰，希望能够帮助大家解决疑惑，下面让小编带领大家一起研究并学习一下“MySQL5.7中select半连接优化的示例分析”这篇文章吧。 MySQL的子查询一直以来都是性能差的著称，解决方法是以join代替。 MySQL5.5版本中该查询先把accessLog表中版本为2.2的数据线过滤出来，然后每个符合条件的数据都与内表进行一次select id from accessLog_01，因此性能低下。MySQL5.5采取的解决方法是将in重写为exists。 在MySQL5.6/5.7版本中，子查询执行计划是将in/exists重写为join，如下看执行计划： 点击(此处)折叠或打开 mysql> select version(); +------------+ | version() | +------------+ | 5.7.18-log | +------------+ 点击(此处)折叠或打开 mysql> explain select * from accessLog ac where ac.id in (select id from accessLog_01); +----+--------------+--------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+--------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+ | 1 | SIMPLE | <subquery2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | 100.00 | NULL | | 1 | SIMPLE | ac | NULL | eq_ref | PRIMARY | PRIMARY | 8 | <subquery2>.id | 1 | 100.00 | NULL | | 2 | MATERIALIZED | accessLog_01 | NULL | ALL | NULL | NULL | NULL | NULL | 1305 | 100.00 | NULL | +----+--------------+--------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+ 点击(此处)折叠或打开 mysql> explain select * from accessLog ac where exists (select * from accessLog_01); +----+-------------+--------------+------------+------+---------------+------+---------+------+--------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------------+------------+------+---------------+------+---------+------+--------+----------+-------+ | 1 | PRIMARY | ac | NULL | ALL | NULL | NULL | NULL | NULL | 586090 | 100.00 | NULL | | 2 | SUBQUERY | accessLog_01 | NULL | ALL | NULL | NULL | NULL | NULL | 1305 | 100.00 | NULL | +----+-------------+--------------+------------+------+---------------+------+---------+------+--------+----------+-------+ 点击(此处)折叠或打开 mysql> explain select ac.* from accessLog ac join accessLog_01 b on ac.id=b.id; +----+-------------+-------+------------+--------+---------------+---------+---------+--------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+--------+---------------+---------+---------+--------------+------+----------+-------+ | 1 | SIMPLE | b | NULL | ALL | NULL | NULL | NULL | NULL | 1305 | 100.00 | NULL | | 1 | SIMPLE | ac | NULL | eq_ref | PRIMARY | PRIMARY | 8 | xinhost.b.id | 1 | 100.00 | NULL | +----+-------------+-------+------------+--------+---------------+---------+---------+--------------+------+----------+-------+ 办连接查询优化默认开启，通过show variables like 'optimizer_switch' G查询： 点击(此处)折叠或打开 mysql> show variables like 'optimizer_switch' G *************************** 1. row *************************** Variable_name: optimizer_switch Value: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,duplicateweedout=on,subquery_materialization_cost_based=on,use_index_extensions=on,condition_fanout_filter=on,derived_merge=on 1 row in set (0.01 sec) 但是半连接优化只是针对查询，对于DML操作，性能依旧很差。 以上是“MySQL5.7中select半连接优化的示例分析”这篇文章的所有内容，感谢各位的阅读！ （编辑：吕梁站长网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
