MySQL5.7中union all用法的黑科技的圖文代碼說明

union all在MySQL5.6下的表現(xiàn)

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(zhí)行結(jié)果如下圖所示：

從執(zhí)行計(jì)劃來看，是把helei表的查詢結(jié)果和t表的查詢結(jié)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(xiàn)

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(zhí)行結(jié)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(zhí)行結(jié)果如下圖所示：

從執(zhí)行結(jié)果看，無論是MySQL5.7還是MariaDB10.1，都沒有創(chuàng)建臨時表，按照順序，helei表的查詢結(jié)果首先輸出到客戶端，然后t表的查詢結(jié)果再輸出到客戶端。

本文中的優(yōu)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(jié)——

在MySQL5.7/MariaDB10.1中，union all不再創(chuàng)建臨時表，這樣在聯(lián)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。

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