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5分钟了解MySQL5.7union all用法的黑科技

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[数据库(mysql) 所属分类 数据库(mysql) | 发布者 店小二05 | 时间 2016 | 作者 红领巾 ] 0人收藏点击收藏

5分钟了解MySQL5.7union all用法的黑科技
5分钟了解MySQL5.7union all用法的黑科技
mysql5.7union all用法的黑科技 union all在MySQL5.6下的表现

Part1: MySQL5.6.25

[[email protected] ~]# mysql -uroot -p Enter password: Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 2 Server version: 5.6.25-log MySQL Community Server (GPL) Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> select version(); +------------+ | version() | +------------+ | 5.6.25-log | +------------+ 1 row in set (0.26 sec) mysql> explain (select id from helei order by id) union all (select id from t where id=0 order by id); +----+--------------+------------+-------+---------------+--------+---------+------+------+-----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------+------------+-------+---------------+--------+---------+------+------+-----------------+ | 1 | PRIMARY | helei | index | NULL | idx_c1 | 4 | NULL | 5219 | Using index | | 2 | UNION | t | ALL | NULL | NULL | NULL | NULL | 1 | Using where | | NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+------------+-------+---------------+--------+---------+------+------+-----------------+ 3 rows in set (0.00 sec)

可以看出,在MySQL5.6版本中,执行结果如下图所示:


5分钟了解MySQL5.7union all用法的黑科技

从执行计划来看,是把helei表的查询结果和t表的查询结果合并在了一张临时表里,然后输出给客户端。

union all在MySQL5.7/MariaDB10.1下的表现

Part1: MySQL5.7.15

[[email protected] ~]# mysql -uroot -p Enter password: Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 8 Server version: 5.7.15-log MySQL Community Server (GPL) Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> select version(); +------------+ | version() | +------------+ | 5.7.15-log | +------------+ 1 row in set (0.00 sec)、 mysql> explain (select id from helei order by id) union all (select id from t where id=0 order by id); +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-------------+ | 1 | PRIMARY | helei | NULL | index | NULL | idx_c1 | 4 | NULL | 5212 | 100.00 | Using index | | 2 | UNION | t | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)

可以看出,在MySQL5.7版本中,执行结果如下图所示:


5分钟了解MySQL5.7union all用法的黑科技

Part2: MariaDB10.1.16

[[email protected] ~]# /usr/local/mariadb/bin/mysql -uroot -S /tmp/mariadb.sock Welcome to the MariaDB monitor. Commands end with ; or \g. Your MariaDB connection id is 7 Server version: 10.1.16-MariaDB MariaDB Server Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. MariaDB [(none)]> MariaDB [helei]> explain (select id from helei order by id) union all (select id from t where id=0 order by id); +------+-------------+-------+-------+---------------+--------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+-------+---------------+--------+---------+------+------+-------------+ | 1 | PRIMARY | helei | index | NULL | idx_c1 | 4 | NULL | 5198 | Using index | | 2 | UNION | t | ALL | NULL | NULL | NULL | NULL | 1 | Using where | +------+-------------+-------+-------+---------------+--------+---------+------+------+-------------+ 2 rows in set (0.00 sec)

可以看出在MariaDB10.1中,执行结果如下图所示:


5分钟了解MySQL5.7union all用法的黑科技

从执行结果看,无论是MySQL5.7还是MariaDB10.1,都没有创建临时表,按照顺序,helei表的查询结果首先输出到客户端,然后t表的查询结果再输出到客户端。

本文中的优化只针对union all,对union和在最外层使用order by无效。如下图是所示:


5分钟了解MySQL5.7union all用法的黑科技

总结

在MySQL5.7/MariaDB10.1中,union all不再创建临时表,这样在联合查询时会减少I/O开销,在MySQL5.5/5.6中则不具备这一特性。由于笔者的水平有限,编写时间也很仓促,文中难免会出现一些错误或者不准确的地方,不妥之处恳请读者批评指正。

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