无用索引导致数据库慢查询问题处理
问题排查
- 新接手的一个网站CPU跑满了，通过
top发现是mysql异常，show processlist查看发现是大量的慢SQL到账的SELECT COUNT(*) FROM idmaps WHERE typeid=8 AND keyid=1085287221 AND showtype=0 , 查下表数据千万量级，ibd文件20几个G，数据不小呀，盘它 - 垃圾代码害死人啊，分析下问题，顺便填坑
- 检查表中索引
show create table idmaps;
CREATE TABLE `idmaps` (
`typeid` tinyint(3) unsigned NOT NULL DEFAULT '0',
`keyid` int(10) unsigned NOT NULL DEFAULT '0',
`myid` int(10) unsigned NOT NULL DEFAULT '0',
`showtype` tinyint(3) unsigned NOT NULL DEFAULT '0',
`exttype` int(10) unsigned NOT NULL DEFAULT '0',
`sortnum` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`typeid`,`myid`,`keyid`),
KEY `idx_list` (`typeid`,`keyid`,`showtype`,`sortnum`),
KEY `idx_list2` (`typeid`,`keyid`,`showtype`,`exttype`,`sortnum`) USING BTREE,
KEY `idx_ids` (`typeid`,`keyid`),
KEY `typeid` (`typeid`),
KEY `keyid` (`keyid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
- 检查索引
explain SELECT COUNT(*) FROM idmaps WHERE typeid=8 AND keyid=275787765 AND showtype=0;
+----+-------------+--------+------+-------------------------------------------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+-------------------------------------------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | idmaps | ref | PRIMARY,idx_list,idx_list2,idx_ids,typeid,keyid | PRIMARY | 1 | const | 1 | Using where |
+----+-------------+--------+------+-------------------------------------------------+---------+---------+-------+------+-------------+
- 莫名其妙的PRIMARY KEY，执行时间20s+, 访问量起来后，基本直接卡死
mysql> SELECT COUNT(*) FROM idmaps WHERE typeid=8 AND keyid=275787765 AND showtype=0;
+----------+
| COUNT(*) |
+----------+
| 2 |
+----------+
1 row in set (21.80 sec)
mysql> SELECT COUNT(*) FROM idmaps force index(idx_ids) WHERE typeid=8 AND keyid=275785755 AND showtype=0;
+----------+
| COUNT(*) |
+----------+
| 0 |
+----------+
1 row in set (0.00 sec)
mysql> explain SELECT COUNT(*) FROM idmaps force index(idx_list) WHERE typeid=8 AND keyid=275787765 AND showtype=0;
+----+-------------+--------+------+---------------+----------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+----------+---------+-------------------+------+-------------+
| 1 | SIMPLE | idmaps | ref | idx_list | idx_list | 6 | const,const,const | 2 | Using index |
+----+-------------+--------+------+---------------+----------+---------+-------------------+------+-------------+
1 row in set (0.00 sec)
mysql> SELECT COUNT(*) FROM idmaps force index(keyid) WHERE typeid=8 AND keyid=275785755 AND showtype=0;
+----------+
| COUNT(*) |
+----------+
| 0 |
+----------+
1 row in set (0.00 sec)
分析过程
- 导致此问题的原因明显是索引不当，结合数据库实际读写情况，检查SQL统计，
PRIMARY KEY基本无用，尝试调整索引解决
处理结果
- 经尝试，最简单办法直接删除
alter table idmaps drop PRIMARY KEY;。 成功解决此慢查询问题，且不影响其它SQL预计。