union all在MySQL5.6下的表現
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，執行結果如下圖所示：
從執行計劃來看，是把helei表的查詢結果和t表的查詢結果合并在了一張臨時表里，然后輸出給客戶端。
union all在MySQL5.7/MariaDB10.1下的表現
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，執行結果如下圖所示：
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，執行結果如下圖所示：
從執行結果看，無論是MySQL5.7還是MariaDB10.1，都沒有創建臨時表，按照順序，helei表的查詢結果首先輸出到客戶端，然后t表的查詢結果再輸出到客戶端。
本文中的優化只針對union all，對union和在最外層使用order by無效。如下圖是所示：
——總結——
在MySQL5.7/MariaDB10.1中，union all不再創建臨時表，這樣在聯合查詢時會減少I/O開銷，在MySQL5.5/5.6中則不具備這一特性。
以上所述是小編給大家介紹的5分鐘了解MySQL5.7中union all用法的黑科技，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對腳本之家網站的支持！
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