MySQL的explain是各种执行计划选择的结果，如果想看整个执行计划以及对于多种索引方案之间是如何选择的？
MySQL5.6中支持这个功能，optimizer_trace
这个是mysql的参数，默认是关闭的
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
mysql> set optimizer_trace="enabled=on";
Query
OK, 0 rows affected
(0.00 sec)
mysql>
show variables like '%optimizer_trace%';
+------------------------------+----------------------------------------------------------------------------+
|
Variable_name | Value |
+------------------------------+----------------------------------------------------------------------------+
|
optimizer_trace | enabled=on,one_line=off |
|
optimizer_trace_features | greedy_search=on,range_optimizer=on,dynamic_range=on,repeated_subselect=on |
|
optimizer_trace_limit | 1 |
|
optimizer_trace_max_mem_size | 16384 |
|
optimizer_trace_offset | -1 |
+------------------------------+----------------------------------------------------------------------------+
5 rows in set (0.00
sec)
|
具体这么使用呢？
需要设置如下：
1、开启optimizer_trace，默认是关闭的
|
1
|
SET optimizer_trace="enabled=on";
|
2、设置optimizer_trace内存的大小
|
1
|
SET OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000
|
3、explain查询语句
|
1
2
3
4
5
6
7
8
9
10
11
12
13
|
mysql>
explain SELECT * FROM atomuser WHERE `uid`
=28778731 AND ptype
= "photo" LIMIT
0 , 1\G
***************************
1. row ***************************
id:
1
select_type:
SIMPLE
table:
atomuser
type:
ref
possible_keys:
uid
key:
uid
key_len:
15
ref:
const,const
rows:
1
Extra:
Using index condition
1
row in set (0.01
sec)
|
4、查找对于的结果
|
1
|
mysql> select * from information_schema.optimizer_trace\G
|
这个表包括4个字段
|
1
2
3
4
5
6
7
8
9
|
mysql>
show create table information_schema.optimizer_trace\G
***************************
1. row ***************************
Table:
OPTIMIZER_TRACE
Create Table: CREATE TEMPORARY TABLE `OPTIMIZER_TRACE`
(
`QUERY`
longtext NOT NULL,
`TRACE`
longtext NOT NULL,
`MISSING_BYTES_BEYOND_MAX_MEM_SIZE` int(20) NOT NULL DEFAULT '0',
`INSUFFICIENT_PRIVILEGES`
tinyint(1) NOT NULL DEFAULT '0'
)
ENGINE=MyISAM DEFAULT CHARSET=utf8
|
主要看TRACE字段，是json串，json解析结果如下：
包括join_preparation,join_optimization,join_explain
join_preparation
|
1
|
"/*
select#1 */ select `atomuser`.`id` AS `id`,`atomuser`.`uid` AS `uid`,`atomuser`.`ptype` AS `ptype`,`atomuser`.`regtime` AS `regtime` from `atomuser` where ((`atomuser`.`uid` = 28778731) and (`atomuser`.`ptype` = 'photo')) limit 0,1"
|
这个是使用extend看到的结果
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
|
mysql>
explain extended SELECT * FROM atomuser WHERE `uid`
=28778731 AND ptype
= "photo" LIMIT
0 , 1\G
***************************
1. row ***************************
id:
1
select_type:
SIMPLE
table:
atomuser
type:
ref
possible_keys:
uid
key:
uid
key_len:
15
ref:
const,const
rows:
1
filtered:
100.00
Extra:
Using index condition
1
row in set,
1 warning (0.01 sec)
mysql>
show warnings\G
***************************
1. row ***************************
Level:
Note
Code:
1003
Message: /*
select#1 */ select `test`.`atomuser`.`id` AS `id`,`test`.`atomuser`.`uid` AS `uid`,`test`.`atomuser`.`ptype` AS `ptype`,`test`.`atomuser`.`regtime` AS `regtime` from `test`.`atomuser` where ((`test`.`atomuser`.`ptype`
= 'photo') and (`test`.`atomuser`.`uid`
= 28778731)) limit 0,1
1
row in set (0.00
sec)
|