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MySQL5.7union all用法的黑科技

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 | | 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：

wKioL1f8bZvhzEMaAAFulp6pefo997.jpg

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：

wKiom1f8bijj3fJiAAF48HG3WPQ918.jpg

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：

wKioL1f8bmmwi9GLAAFbMJCN0uU554.jpg

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：

wKiom1f8boazPx35AAKnKQS1Ig4776.jpg

——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。由于筆者的水平有限，編寫時間也很倉促，文中難免會出現一些錯誤或者不準確的地方，不妥之處懇請讀者批評指正。

?著作權歸作者所有：來自51CTO博客作者dbapower的原創作品，如需轉載，請注明出處，否則將追究法律責任

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總結

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