Mysql应用5分钟了解MySQL5.7中union all用法的黑科技

《Mysql应用5分钟了解MySQL5.7中union all用法的黑科技》要点：
本文介绍了Mysql应用5分钟了解MySQL5.7中union all用法的黑科技，希望对您有用。如果有疑问，可以联系我们。

union all在MySQL5.6下的表现MYSQL学习

Part1:MySQL5.6.25MYSQL学习

[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：MYSQL学习

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.
MYSQL学习

union all在MySQL5.7/MariaDB10.1下的表现
MYSQL学习

Part1:MySQL5.7.15
MYSQL学习

[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：
MYSQL学习

Part2:MariaDB10.1.16MYSQL学习

[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,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：MYSQL学习

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.
MYSQL学习

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：?MYSQL学习

――总结――MYSQL学习

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.MYSQL学习

以上所述是小编给大家介绍的5分钟了解MySQL5.7中union all用法的黑科技,希望对大家有所帮助,如果大家有任何疑问请给我留言,小编会及时回复大家的.在此也非常感谢大家对编程之家PHP网站的支持!MYSQL学习

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