MySQL5.7中union all用法的黑科技的图文代码介绍

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：

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：

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：

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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