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我们使用MySql 5.5.37，Java 7和Hibernate 5.1.3。 Hibernate是自动生成查询，有一个让我感到困惑。我们有这些表...为什么MySql没有在我们的表上使用索引？
CREATE TABLE `user` (
`ID` varchar(32) COLLATE utf8_bin NOT NULL,
`FIRST_NAME` varchar(50) COLLATE utf8_bin NOT NULL,
`MIDDLE_NAME` varchar(50) COLLATE utf8_bin DEFAULT NULL,
`LAST_NAME` varchar(50) COLLATE utf8_bin NOT NULL,
`USER_NAME` varchar(50) COLLATE utf8_bin NOT NULL,
`URL` varchar(200) COLLATE utf8_bin NOT NULL,
`SALUTATION` varchar(10) COLLATE utf8_bin DEFAULT NULL,
...
`ADDRESS_ID` varchar(32) COLLATE utf8_bin DEFAULT NULL,
PRIMARY KEY (`ID`),
UNIQUE KEY `USER_IDX_01` (`USER_NAME`,`URL`),
KEY `FK2_USER` (`GRADE_ID`),
KEY `FK4_USER` (`USER_DEMOGRAPHIC_INFO_ID`),
KEY `FK3_USER` (`CREATOR_ID`),
KEY `FK_USER` (`ADDRESS_ID`),
CONSTRAINT `FK2_USER` FOREIGN KEY (`GRADE_ID`) REFERENCES `grade` (`ID`),
CONSTRAINT `FK3_USER` FOREIGN KEY (`CREATOR_ID`) REFERENCES `user` (`ID`) ON UPDATE NO ACTION,
CONSTRAINT `FK_USER` FOREIGN KEY (`ADDRESS_ID`) REFERENCES `cb_address` (`ID`) ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin |
和
role | CREATE TABLE `role` (
`ID` varchar(40) COLLATE utf8_bin NOT NULL,
`NAME` varchar(40) COLLATE utf8_bin NOT NULL,
PRIMARY KEY (`ID`),
UNIQUE KEY `ROLE_IDX_01` (`NAME`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin |
，我们有一个连接表将它们连接在一起
user_role | CREATE TABLE `user_role` (
`USER_ID` varchar(32) COLLATE utf8_bin NOT NULL,
`ROLE_ID` varchar(40) COLLATE utf8_bin NOT NULL,
UNIQUE KEY `USER_ROLE_IDX_02` (`USER_ID`,`ROLE_ID`),
KEY `FK2_USER_ROLE` (`ROLE_ID`),
CONSTRAINT `FK1_USER_ROLE` FOREIGN KEY (`USER_ID`) REFERENCES `user` (`ID`) ON DELETE CASCADE ON UPDATE NO ACTION,
CONSTRAINT `FK2_USER_ROLE` FOREIGN KEY (`ROLE_ID`) REFERENCES `role` (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin |
但是，当我们查询一个用户，包括在角色我们的查询MySql似乎忽略了索引。请注意解释计划中的“1688568”行。为什么MySQL不使用索引以及哪种方式更好地查询数据？
mysql> EXPLAIN select user0_.id as id1_112_, user0_.ADDRESS_ID as ADDRESS17_112_,
user0_.AVATAR as AVATAR2_112_, user0_.CREATED_ON as CREATED_3_112_,
user0_.CREATOR_ID as CREATOR18_112_, user0_.DOB as DOB4_112_,
user0_.ENABLED as ENABLED5_112_, user0_.EXPIRATION as EXPIRATI6_112_,
user0_.first_name as first_na7_112_, user0_.GRADE_ID as GRADE_I19_112_,
user0_.INCORRECT_LOGINS as INCORREC8_112_, user0_.last_name as last_nam9_112_,
user0_.middle_name as middle_10_112_, user0_.password as passwor11_112_,
user0_.RESET_STATE as RESET_S12_112_, user0_.salutation as salutat13_112_,
user0_.temporary_password as tempora14_112_, user0_.url as url15_112_,
user0_.USER_DEMOGRAPHIC_INFO_ID as USER_DE20_112_, user0_.user_name as user_na16_112_
from user user0_
inner join user_role roles1_ ON user0_.id = roles1_.USER_ID
inner join role role2_ ON roles1_.ROLE_ID = role2_.ID
inner join cb_address address3_ ON user0_.ADDRESS_ID = address3_.id
where user0_.url = 'mycityst.myco.org'
and (role2_.ID in ('Student'))
and lower(address3_.email) = '[email protected]'
and user0_.ENABLED = 1;
+----+-------------+-----------+--------+--------------------------------------+------------------+---------+---------------------------+---------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+--------+--------------------------------------+------------------+---------+---------------------------+---------+--------------------------+
| 1 | SIMPLE | role2_ | const | PRIMARY | PRIMARY | 122 | const | 1 | Using index |
| 1 | SIMPLE | roles1_ | ref | USER_ROLE_IDX_02,FK2_USER_ROLE | FK2_USER_ROLE | 122 | const | 1688568 | Using where; Using index |
| 1 | SIMPLE | user0_ | eq_ref | PRIMARY,FK_USER | PRIMARY | 98 | schema1.roles1_.USER_ID | 1 | Using where |
| 1 | SIMPLE | address3_ | eq_ref | PRIMARY | PRIMARY | 98 | schema1.user0_.ADDRESS_ID | 1 | Using where |
+----+-------------+-----------+--------+--------------------------------------+------------------+---------+---------------------------+---------+--------------------------+