MySQL5.6的optimizer_trace

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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                                                                         |
+------------------------------+----------------------------------------------------------------------------+
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

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SET optimizer_trace="enabled=on";  

 

2、设置optimizer_trace内存的大小

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SET OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000

  

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)

  

4、查找对于的结果

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mysql> select from information_schema.optimizer_trace\\G

 

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 TableCREATE 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

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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"

 

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)

  

[参考资料]

1、http://guilhembichot.blogspot.com/2011/09/optimizer-tracing-how-to-configure-it.html

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