标签：NULL 正确 mmmoverttme 索引 sn MySQL mmmtransttme devabce id
发现有个mysql库的cpu使用比较厉害，分析后发现是以下的sql导致：
SELECT `devabce`.`id`, `devabce`.`sn_id`, `devabce`.`mmmcontent`, `devabce`.`mmmcommitttme`, `devabce`.`mmmtransttme`, `devabce`.`mmmoverttme`, `devabce`.`mmmreturn`, `devabce`.`reserved` FROM `devabce` WHERE ( `devabce`.`sn_id` = 'ADBG182760058' AND (( `devabce`.`mmmoverttme` IS NULL AND `devabce`.`mmmtransttme` IS NULL ) OR ( `devabce`.`mmmoverttme` IS NULL AND `devabce`.`mmmtransttme` IS NOT NULL AND `devabce`.`mmmtransttme` > '2020-04-18 18:40:40.466628' ))) ORDER BY `devabce`.`id` ASC LIMIT 200;
该语句单次执行时间长达376.424870秒。而且是执行比较频繁的语句。
查看该sql的执行计划：
+----+-------------+---------+------------+-------+----------------------------------------------------------+---------+---------+------+-------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------------------------------------------------+---------+---------+------+-------+----------+-------------+ | 1 | SIMPLE | devabce | NULL | index | devabce_sn_id_c2b22ec1_fk_iclock_sn,idx_snid_ottme_tttme | PRIMARY | 4 | NULL | 59125 | 0.07 | Using where | +----+-------------+---------+------------+-------+----------------------------------------------------------+---------+---------+------+-------+----------+-------------+
从执行计划可以看到，优化器选择使用了主键。
查看该表，表中有数据2千多万条；索引idx_snid_ottme_tttme是建立在列(sn_id、mmmoverttme、mmmtransttme)，而且该索引的选择性很好。按道理应该使用该索引，而不是使用主键。
首先尝试对表analyze，然后再次执行，发现还是使用了主键。索引仍然没有选对。鉴于目前不方便对表执行optimze。就尝试使用hint，看看执行效果。
1.使用use index(idx_snid_ottme_tttme)
执行大约0.3秒即可返回数据。执行计划如下：
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+-------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+-------+----------+---------------------------------------+ | 1 | SIMPLE | devabce | NULL | range | idx_snid_ottme_tttme | idx_snid_ottme_tttme | 100 | NULL | 84826 | 100.00 | Using index condition; Using filesort | +----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+-------+----------+---------------------------------------+
2.修改原sql，使得不走主键
SELECT `devabce`.`id`, `devabce`.`sn_id`, `devabce`.`mmmcontent`, `devabce`.`mmmcommitttme`, `devabce`.`mmmtransttme`, `devabce`.`mmmoverttme`, `devabce`.`mmmreturn`, `devabce`.`reserved` FROM `devabce` WHERE ( `devabce`.`sn_id` = 'ADBG182760058' AND (( `devabce`.`mmmoverttme` IS NULL AND `devabce`.`mmmtransttme` IS NULL ) OR ( `devabce`.`mmmoverttme` IS NULL AND `devabce`.`mmmtransttme` IS NOT NULL AND `devabce`.`mmmtransttme` > '2020-04-18 18:40:40.466628' ))) ORDER BY `devabce`.`id`+0 ASC LIMIT 200;
执行0.3秒即可返回数据，执行计划如下：选对了索引
+----+-------------+---------+------------+-------+----------------------------------------------------------+----------------------+---------+------+-------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------------------------------------------------+----------------------+---------+------+-------+----------+---------------------------------------+ | 1 | SIMPLE | devabce | NULL | range | devabce_sn_id_c2b22ec1_fk_iclock_sn,idx_snid_ottme_tttme | idx_snid_ottme_tttme | 100 | NULL | 84826 | 100.00 | Using index condition; Using filesort | +----+-------------+---------+------------+-------+----------------------------------------------------------+----------------------+---------+------+-------+----------+---------------------------------------+
3.使用IGNORE INDEX(PRIMARY)
SELECT `devabce`.`id`, `devabce`.`sn_id`, `devabce`.`mmmcontent`, `devabce`.`mmmcommitttme`, `devabce`.`mmmtransttme`, `devabce`.`mmmoverttme`, `devabce`.`mmmreturn`, `devabce`.`reserved` FROM `devabce` IGNORE INDEX(PRIMARY) WHERE ( `devabce`.`sn_id` = 'ADBG182760058' AND (( `devabce`.`mmmoverttme` IS NULL AND `devabce`.`mmmtransttme` IS NULL ) OR ( `devabce`.`mmmoverttme` IS NULL AND `devabce`.`mmmtransttme` IS NOT NULL AND `devabce`.`mmmtransttme` > '2020-04-18 18:40:40.466628' ))) ORDER BY `devabce`.`id` ASC LIMIT 200;
执行了大约4秒，从执行计划看，虽然没有使用主键，但是仍然选错了索引
+----+-------------+---------+------------+-------+----------------------------------------------------------+-------------------------------------+---------+------+--------+----------+------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------------------------------------------------+-------------------------------------+---------+------+--------+----------+------------------------------------+ | 1 | SIMPLE | devabce | NULL | range | devabce_sn_id_c2b22ec1_fk_iclock_sn,idx_snid_ottme_tttme | devabce_sn_id_c2b22ec1_fk_iclock_sn | 82 | NULL | 429784 | 3.97 | Using index condition; Using where | +----+-------------+---------+------------+-------+----------------------------------------------------------+-------------------------------------+---------+------+--------+----------+------------------------------------+
删除索引devabce_sn_id_c2b22ec1_fk_iclock_sn，再次测试上面的语句，就可以选对索引了。
针对mysql不能选对正确索引，查了一下官方信息，发现有个类似的bug：
暂时怀疑是该bug所导致。
因为不是自己的库，还得等待下周同事来确认。
标签：NULL,正确,mmmoverttme,索引,sn,MySQL,mmmtransttme,devabce,id 来源： https://www.cnblogs.com/abclife/p/12731060.html
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