5分钟了解MySQL5.7中union all用法的黑科技

union all在MySQL5.6下的表现

Part1:MySQL5.6.25

[root@HE1 ~]# MySQL -uroot -pEnter password: Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 2Server 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 itsaffiliates. Other names may be trademarks of their respectiveowners.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)

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

[root@HE1 ~]# mysql -uroot -pEnter password: Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 8Server 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 itsaffiliates. Other names may be trademarks of their respectiveowners.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)

Part2:MariaDB10.1.16

[root@HE3 ~]# /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 7Server version: 10.1.16-MariaDB MariaDB ServerCopyright (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)

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：

——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。

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