5分钟了解MySQL5.7union all用法的黑科技

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MySQL5.7union all用法的黑科技

union all在MySQL5.6下的表现

Part1:MySQL5.6.25

[root@HE1 ~]# 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：

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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 -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：

wKiom1f8bijj3fJiAAF48HG3WPQ918.jpg

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 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：

wKioL1f8bmmwi9GLAAFbMJCN0uU554.jpg

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：

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——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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