Question

当以条件作为索引列自加入表时，mysql会进行全表扫描。通过更改连接顺序，我们可以使用索引。

EmployeeDetails的表架构

EmployeeDetails | CREATE TABLE `EmployeeDetails` (
  `CompanyId` bigint(19) NOT NULL DEFAULT '0',
  `EmployeeId` bigint(19) NOT NULL DEFAULT '0',
  `ParentEmployeeId` bigint(19) DEFAULT NULL,
  PRIMARY KEY (`EmployeeId`),
  KEY `EmployeeDetails_FK1_IDX` (`CompanyId`),
  KEY `EmployeeDetails_FK2_IDX` (`ParentEmployeeId`),
  CONSTRAINT `EmployeeDetails_FK1` FOREIGN KEY (`CompanyId`) REFERENCES `CompanyDetails` (`CompanyId`) ON DELETE CASCADE,
  CONSTRAINT `EmployeeDetails_FK2` FOREIGN KEY (`ParentEmployeeId`) REFERENCES `EmployeeDetails` (`EmployeeId`) ON DELETE CASCADE,
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

CompanyDetails的表架构

| CompanyDetails | CREATE TABLE `CompanyDetails` (
  `CompanyId` bigint(19) NOT NULL DEFAULT '0',
  `NAME` varchar(25) NOT NULL DEFAULT '',
  `DESCRIPTION` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`CompanyId`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 
|

第一个查询-第一次选择将扫描EmployeeDetails表中的所有记录并进行自连接（不使用在条件中使用的索引KEY EmployeeDetails_FK1_IDX（CompanyId））

mysql> select * from EmployeeDetails INNER JOIN `EmployeeDetails` `t1` ON `EmployeeDetails`.`EmployeeId`=`t1`.`ParentEmployeeId` where `EmployeeDetails`.`CompanyId` IN(123);
Empty set (0.10 sec)

mysql> explain select * from EmployeeDetails INNER JOIN `EmployeeDetails` `t1` ON `EmployeeDetails`.`EmployeeId`=`t1`.`ParentEmployeeId` where `EmployeeDetails`.`CompanyId` IN(123)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: ALL
possible_keys: EmployeeDetails_FK2_IDX
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 7999
     filtered: 100.00
        Extra: Using where
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: EmployeeDetails
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY,EmployeeDetails_FK1_IDX
          key: PRIMARY
      key_len: 8
          ref: db.t1.ParentEmployeeId
         rows: 1
     filtered: 5.00
        Extra: Using where
2 rows in set, 1 warning (0.01 sec)

第二个查询-首先使用EmployeeDetails_FK1_IDX（CompanyId`）索引从EmployeeDetails中选择记录，然后仅扫描符合条件的记录并进行自我加入

mysql> select * from EmployeeDetails INNER JOIN `EmployeeDetails` `t1` ON `t1`.`EmployeeId`=`EmployeeDetails`.`ParentEmployeeId` where `EmployeeDetails`.`CompanyId` IN(123);
Empty set (0.00 sec)

mysql> explain select * from EmployeeDetails INNER JOIN `EmployeeDetails` `t1` ON `t1`.`EmployeeId`=`EmployeeDetails`.`ParentEmployeeId` where `EmployeeDetails`.`CompanyId` IN(123)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: EmployeeDetails
   partitions: NULL
         type: ref
possible_keys: EmployeeDetails_FK1_IDX,EmployeeDetails_FK2_IDX
          key: EmployeeDetails_FK1_IDX
      key_len: 8
          ref: const
         rows: 54
     filtered: 100.00
        Extra: Using where
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 8
          ref: db.EmployeeDetails.ParentEmployeeId
         rows: 1
     filtered: 100.00
        Extra: NULL
2 rows in set, 1 warning (0.00 sec)

为什么会这样？

Answer 1

从说明中可以看出，“ EmployeeDetails_FK2_IDX”被列为可能的键。如果MySQL优化器不使用它，那只会减少查询时间。

尝试用数据填充表（我检查了它，它按原样工作），您应该看到MySQL优化器将使用索引进行连接，而无需查看其顺序。因为连接的顺序对MySQL优化器算法没有任何影响。

还要选中 STRAIGHT_JOIN

Mysql在使用索引条件自联接表时不使用索引

1 个答案: