explain analysis of the implementation plan
Locate the problem through the above steps sql statements in the future we can explain to analyze the select execution plan statement , including the order of how the statement connections and connections
Explain select * from user where id = 1;
This command can display select * from user where id = 1 execution plan of the query
Id: select the query sequence number , represents select the order of lookup tables.
Select_type: represents a select type , common values have the SIMPLE ( simple table , i.e., without connecting the table and subqueries ), a PRIMARY ( main query ) that is the outermost subquery, the UNION (Union second query or behind the query ), sUBQUERY (the first sub-query the SELECT ), etc.
Table: Table query
Type: the type of connection performance table is from good to bad : system-> const-> eq_ref-> ref- > ref_or_null-> index_merge-> index_subquery-> range-> index-> all
possible_keys: may be used when the query index
key: the index of actual use
: key_len length of the index field
: rows number of scan lines
extra: and described implementation
Below through an example elaborate on:
Create a user table, table roles, user roles correspondence table and insert data
CREATE TABLE `t_role` (
`id` varchar(32) NOT NULL,
`role_name` varchar(255) DEFAULT NULL,
`role_code` varchar(255) DEFAULT NULL,
`description` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `unique_role_name` (`role_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `t_user` (
`id` varchar(32) NOT NULL,
`username` varchar(45) NOT NULL,
`password` varchar(96) NOT NULL,
`name` varchar(45) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `unique_user_username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `user_role` (
`id` int(11) NOT NULL auto_increment ,
`user_id` varchar(32) DEFAULT NULL,
`role_id` varchar(32) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_ur_user_id` (`user_id`),
KEY `fk_ur_role_id` (`role_id`),
CONSTRAINT `fk_ur_role_id` FOREIGN KEY (`role_id`) REFERENCES `t_role` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `fk_ur_user_id` FOREIGN KEY (`user_id`) REFERENCES `t_user` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
insert into `t_user` (` id`, `username`,` password`, `name`) values ( '1', 'super', '$ 2a $ 10 $ TJ4TmCdK.X4wv / tCqHW14.w70U3CC33CeVncD3SLmyMXMknstqKRe', ' super administrator ');
insert into `t_user` (` id`, `username`,` password`, `name`) values ( '2', 'admin', '$ 2a $ 10 $ TJ4TmCdK.X4wv / tCqHW14.w70U3CC33CeVncD3SLmyMXMknstqKRe', ' system administrators ');
insert into `t_user` (`id`, `username`, `password`, `name`) values('3','itcast','$2a$10$8qmaHgUFUAmPR5pOuWhYWOr291WJYjHelUlYn07k5ELF8ZCrW0Cui','test02');
insert into `t_user` (`id`, `username`, `password`, `name`) values('4','stu1','$2a$10$pLtt2KDAFpwTWLjNsmTEi.oU1yOZyIn9XkziK/y/spH5rftCpUMZa','学生1');
insert into `t_user` (`id`, `username`, `password`, `name`) values('5','stu2','$2a$10$nxPKkYSez7uz2YQYUnwhR.z57km3yqKn3Hr/p1FR6ZKgc18u.Tvqm','学生2');
insert into `t_user` (`id`, `username`, `password`, `name`) values('6','t1','$2a$10$TJ4TmCdK.X4wv/tCqHW14.w70U3CC33CeVncD3SLmyMXMknstqKRe','老师1');
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('5','学生','student','学生');
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('7','老师','teacher','老师');
INSERT INTO `t_role` (` id`, `role_name`,` role_code`, `description`) VALUES ( '8', ' teaching administrators ', 'teachmanager', ' teaching administrators ');
INSERT INTO `t_role` (` id`, `role_name`,` role_code`, `description`) VALUES ( '9', ' Administrators ', 'admin', ' Administrators ');
INSERT INTO `t_role` (` id`, `role_name`,` role_code`, `description`) VALUES ('10 ',' super administrator ',' super ',' super administrator ');
INSERT INTO user_role(id,user_id,role_id) VALUES(NULL, '1', '5'),(NULL, '1', '7'),(NULL, '2', '8'),(NULL, '3', '9'),(NULL, '4', '8'),(NULL, '5', '10') ;
Explain the id:
. 1) ID in the same order from top to bottom of the table represents a load
explain select * from t_role r, t_user u, user_role ur where r.id = ur.role_id and u.id = ur.user_id ;
Here the first load t_role table reloading t_user, loaded last user_role
2) id different id higher the value, the higher the priority the earlier it is performed
EXPLAIN SELECT * FROM t_role WHERE id = (SELECT role_id FROM user_role WHERE user_id = (SELECT id FROM t_user WHERE username = 'stu1'))
Here t_user first to be loaded and then user_role, finally t_role
. 3) ID have the same or different, exist. id same can be considered as a group, from the order of execution down; in all groups, id larger value, the higher the priority, the first run.
EXPLAIN SELECT * FROM t_role r , (SELECT * FROM user_role ur WHERE ur.`user_id` = '2') a WHERE r.id = a.role_id ;
explain 之 select_type：
It represents select the type of the following values:
SIMPLE: simple query is, do not contain sub-queries and union
PRIMARY: If a query contains subqueries that the parent query with the flag
SUBQUERY: express subqueries
DERIVED: representation from from the temporary table query query results in the formation of the child out of the query
UNION: joint query UNION after the select statement
UNIONRESULT: by UNION joint statement