MySQL5.6的optimizer_trace

Posted 2020-09-25 旅途

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)

　　

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

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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

1	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 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　

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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