MySQL5.6的optimizer_trace
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
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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)
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具体这么使用呢？
需要设置如下：
1、开启optimizer_trace，默认是关闭的
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SET optimizer_trace="enabled=on";
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2、设置optimizer_trace内存的大小
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SET OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000
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3、explain查询语句
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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)
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4、查找对于的结果
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mysql> select * from information_schema.optimizer_trace\G
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这个表包括4个字段
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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
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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
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"/* 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"
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这个是使用extend看到的结果
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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)
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