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

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

SET optimizer_trace="enabled=on";

　

2、设置optimizer_trace内存的大小

SET OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000

　　

3、explain查询语句

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

mysql> select * from information_schema.optimizer_trace\G

　

这个表包括4个字段　

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

"/* 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　

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