MySQL5.7中union all用法的黑科技的圖文程式碼介紹

本文帶領大家透過5分鐘了解MySQL5.7中union all用法的黑科技,需要的朋友可以參考下

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表的查詢結果合併在了一張臨時表裡，然後輸出給客戶端。

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：

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：

從執行結果來看，無論是MySQL5.7或MariaDB10.1，都沒有建立臨時表，依照順序，helei表的查詢結果先輸出到客戶端，然後t表的查詢結果再輸出到客戶端。

本文中的最佳化只針對union all，對union和在最外層使用order by無效。如下圖所示： 

—總結—

在MySQL5.7/MariaDB10.1中，union all不再建立臨時表，這樣在聯合查詢時會減少I/O開銷，在MySQL5.5/5.6中則不具備此特性。

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