Part1:MySQL5.6.25
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[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。
Part1:MySQL5.7.15
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[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
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[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。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
原文：乐搏学院http://www.learnbo.com/
