1

我无法使用 MySQL InnoDB 优化器优化请求。以下查询(查询 1)高效运行:

explain select * from ah_problems
where rnid in (6022342, 6256614, 5842714, 6302489)
and fieldid in (5,6);

计划(计划一)如下:

id select_type table       type  possible_keys                   key           key_len ref rows Extra
=  ======      =========== ===== =============================== ============= ======= === ==== =====
1  SIMPLE      ah_problems range CONSTRAINTFIELDID,RNID__FIELDID RNID__FIELDID 8           33   Using where

到现在为止还挺好。

而下面稍作修改的查询(查询 2)将采用灾难性的执行计划:

explain select * from ah_problems
where rnid in (select rec.rnid as record_id from ar_records rec where rnid in (6022342, 6256614, 5842714, 6302489))
and fieldid in (5, 6)

结果是一样的,但是计划(计划2)现在是这样做的:

id select_type        table       type            possible_keys      key      key_len ref  rows     Extra
=  ======             =========== =====           ================== ======== ======= ==== =======  =====
1  PRIMARY            ah_problems ALL             CONSTRAINTFIELDID                        36177754 Using where
2  DEPENDENT SUBQUERY rec         unique_subquery PRIMARY            PRIMARY  4       func 1        Using index; Using where

如果您想知道,那个新的子查询...

select rec.rnid as record_id from ar_records rec where rnid in (6022342, 6256614, 5842714, 6302489)

...只不过返回在查询 1 中硬编码的四行:

6022342
6256614
5842714
6302489

所以查询 (1) 和 (2) 是等价的。

你猜怎么着,我需要查询 2,而不是一个。我希望查询 2 与查询 1 一样高效。我尝试了以下方法:

  1. 查询 3:添加FORCE INDEX(RNID_FIELDID)到查询 2。MySQL 直接忽略它。

    解释 select * from ah_problems force index (rnid__fieldid) where rnid in (select rec.rnid as record_id from ar_records rec where rnid in (6022342, 6256614, 5842714, 6302489)) 和 fieldid in (5,6)

执行计划与计划2相同。

  1. 查询 4:ORDER BY RNID, FIELDID向查询 3 添加一个。我在其他一些问题上看到这可能会欺骗优化器。它没有帮助。

    解释 select * from ah_problems force index (rnid__fieldid) where rnid in (select rec.rnid as record_id from ar_records rec where rnid in (6022342, 6256614, 5842714, 6302489)) and fieldid in (5, 6) order by rnid, fieldid

计划 4 现在正在使用索引,但行数仍然是灾难性的:

id select_type        table       type            possible_keys      key           key_len ref  rows      Extra
=  ======             =========== =====           ================== ========      ======= ==== =======   =====
1  PRIMARY            ah_problems index                              RNID__FIELDID 8             36179307 Using where
2  DEPENDENT SUBQUERY rec         unique_subquery PRIMARY            PRIMARY       4       func  1        Using index; Using where

如果这有帮助,这就是我的ah_problems表格的定义。不幸的是,我无法更改表格的定义。我能做些什么来让 MySQL 优化器使用计划 1 来攻击ah_problems查询 2 中的表吗?

CREATE TABLE `ah_problems` (
  `ID` int(11) NOT NULL AUTO_INCREMENT COMMENT 'Identifier for update statements',
  `RNID` int(11) NOT NULL COMMENT 'Record number',
  `FIELDID` int(11) NOT NULL COMMENT 'Which field is value in',
  `VALUE` varchar(255) NOT NULL COMMENT 'The value the field got on MODIFIED_DATE',
  `PREVIOUSID` int(11) DEFAULT NULL COMMENT 'Reference to previous value',
  `MODIFIED_DATE` datetime NOT NULL COMMENT 'When was it changed',
  `MODIFIED_GROUPID` int(11) DEFAULT NULL COMMENT 'In what group did modified_userid change it',
  `MODIFIED_USERID` int(11) NOT NULL COMMENT 'Who changed it',
  PRIMARY KEY (`ID`),
  KEY `CONSTRAINTFIELDID` (`FIELDID`),
  KEY `CONSTRAINTMODIFIED_GROUPID` (`MODIFIED_GROUPID`),
  KEY `CONSTRAINTMODIFIED_USERID` (`MODIFIED_USERID`),
  KEY `CONSTRAINTPREVIOUSID` (`PREVIOUSID`),
  KEY `RNID__FIELDID` (`RNID`,`FIELDID`),
  CONSTRAINT `HPRB_FIELD` FOREIGN KEY (`FIELDID`) REFERENCES `ad_fields` (`ID`),
  CONSTRAINT `HPRB_MODIFIED_GROUP` FOREIGN KEY (`MODIFIED_GROUPID`) REFERENCES `ap_groups` (`ID`),
  CONSTRAINT `HPRB_MODIFIED_USER` FOREIGN KEY (`MODIFIED_USERID`) REFERENCES `ap_users` (`ID`),
  CONSTRAINT `HPRB_PREVIOUS` FOREIGN KEY (`PREVIOUSID`) REFERENCES `ah_problems` (`ID`) ON DELETE CASCADE,
  CONSTRAINT `HPRB_RN` FOREIGN KEY (`RNID`) REFERENCES `ar_records` (`RNID`)
) ENGINE=InnoDB AUTO_INCREMENT=72305308 DEFAULT CHARSET=utf8 COMMENT='PTR history'$$
4

2 回答 2

1

MySQL无法将IN子查询优化为领先(仅执行一次),它总是在循环中为主查询中的每条记录执行。

将其替换为连接:

SELECT ahp.*
FROM   ar_records ar
JOIN   ah_problems ahp
ON     ahp.rnid = ar.rnid
       AND ahp.fieldId IN (5, 6)
WHERE  ar.rnid IN (6022342, 6256614, 5842714, 6302489)
于 2013-04-30T16:46:07.310 回答
1

我相信 MySQL 需要WHERE fieldid IN (5, 6)在子查询之前解决。尝试添加以下索引:

ALTER TABLE ah_problems ADD INDEX FIELDID__RNID (FIELDID, RNID);

如果没有帮助,请将您的查询重写为JOIN

SELECT *
FROM ah_problems
JOIN ar_records USING (rnid)
WHERE rnid in (6022342, 6256614, 5842714, 6302489)
AND fieldid in (5, 6)
于 2013-04-30T16:47:48.603 回答