union all在MySQL5.6下的表现
Part1:MySQL5.6.25
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[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：
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。
union all在MySQL5.7/MariaDB10.1下的表现
Part1:MySQL5.7.15
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[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：
Part2:MariaDB10.1.16
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[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，执行结果如下图所示：
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。
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：
——总结——
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。由于笔者的水平有限，编写时间也很仓促，文中难免会出现一些错误或者不准确的地方，不妥之处恳请读者批评指正。
本文转自 dbapower 51CTO博客，原文链接：http://blog.51cto.com/suifu/1860575，如需转载请自行联系原作者
