一个奇怪的MySQL慢查询,打懵了一群不懂业务的DBA!

前言

最近,开发人员需要定期的删除表里一定时间以前的数据,SQL如下: mysql > delete from testtable WHERE biz_date <= '2017-08-21 00:00:00' AND status = 2 limit 500\G 前段时间在优化的时候,我们已经在相应的查询条件上加上了索引,如下: KEY idx_bizdate_st (biz_date,status) 但是实际执行的SQL依然非常慢,为什么呢,我们来一步步分析验证下。

分析

表上的字段既然都有索引,那么按照之前的文章分析,是两个字段都可以走上索引的。

既然能够利用索引,表的总大小也就是200M左右,那么为什么形成了慢查呢?

我们查看执行计划,去掉limit 后,发现他选择了走全表扫描。

mysql > desc  select * from testtable   WHERE biz_date <= '2017-08-21 00:00:00';
+----+-------------+-----------+------+----------------+------+---------+------+--------+-------------+
| id | select_type | table     | type | possible_keys  | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-----------+------+----------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | testtable | ALL  | idx_bizdate_st | NULL | NULL    | NULL | 980626 | Using where |
+----+-------------+-----------+------+----------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

-- 只查询biz_date
-- 关键点:rows:980626;type:ALL

mysql > desc  select * from testtable   WHERE biz_date <= '2017-08-21 00:00:00' and status = 2;
+----+-------------+-----------+------+----------------+------+---------+------+--------+-------------+
| id | select_type | table     | type | possible_keys  | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-----------+------+----------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | testtable | ALL  | idx_bizdate_st | NULL | NULL    | NULL | 980632 | Using where |
+----+-------------+-----------+------+----------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

-- 查询biz_date + status 
-- 关键点:rows:980632;type:ALL

mysql > desc  select * from testtable   WHERE biz_date <= '2017-08-21 00:00:00' and status = 2 limit 100;
+----+-------------+-----------+-------+----------------+----------------+---------+------+--------+-----------------------+
| id | select_type | table     | type  | possible_keys  | key            | key_len | ref  | rows   | Extra                 |
+----+-------------+-----------+-------+----------------+----------------+---------+------+--------+-----------------------+
|  1 | SIMPLE      | testtable | range | idx_bizdate_st | idx_bizdate_st | 6       | NULL | 490319 | Using index condition |
+----+-------------+-----------+-------+----------------+----------------+---------+------+--------+-----------------------+
1 row in set (0.00 sec)

-- 查询biz_date + status+ limit 
-- 关键点:rows:490319;

mysql > select count(*)  from testtable   WHERE biz_date <= '2017-08-21 00:00:00' and status = 2;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.34 sec)

mysql > select count(*)  from testtable   WHERE biz_date <= '2017-08-21 00:00:00';
+----------+
| count(*) |
+----------+
|   970183 |
+----------+
1 row in set (0.33 sec)

mysql > select count(*)  from testtable;
+----------+
| count(*) |
+----------+
|   991421 |
+----------+
1 row in set (0.19 sec)

mysql > select distinct biz_status from testtable;
+------------+
| biz_status |
+------------+
|          1 |
|          2 |
|          4 |
+------------+

通过以上查询,我们可以发现如下几点问题:

通过 biz_date 预估出来的行数 和 biz_date + status=2 预估出来的行数几乎一样,为98w。 实际查询表 biz_date + status=2 一条记录都没有。 整表数据量达到了99万,MySQL发现通过索引扫描需要98w行(预估)

因此,MySQL通过统计信息预估的时候,发现需要扫描的索引行数几乎占到了整个表,放弃了使用索引,选择了走全表扫描。

那是不是他的统计信息有问题呢?我们重新收集了下表统计信息,发现执行计划的预估行数还是一样,猜测只能根据组合索引的第一个字段进行预估(待确定)。

那我们试下直接强制让他走索引呢?

mysql > select * from testtable   WHERE biz_date <= '2017-08-21 00:00:00' and status = 2;
Empty set (0.79 sec)


mysql > select * from testtable force index(idx_bizdate_st)  WHERE biz_date <= '2017-08-21 00:00:00' and status = 2;
Empty set (0.16 sec)

我们发现,强制指定索引后,查询耗时和没有强制索引比较,的确执行速度快了很多,因为没有强制索引是全表扫描嘛!但是!依然非常慢!

那么还有什么办法去优化这个本来应该很快的查询呢?

大家应该都听说过要选择性好的字段放在组合索引的最前面?

选择性好的索引在前面并不是对所有的场景都通用的,这个场景可以将status放前面,sql速度会更快。

那,能不能让他不要扫描索引的那么多范围呢?之前的索引模型中也说过,MySQL是通过索引去确定一个扫描范围,如果能够定位到尽可能小的范围,那是不是速度上会快很多呢?

并且业务逻辑上是定期删除一定日期之前的数据。所以逻辑上来说,每次删除都是只删除一天的数据,直接让SQL扫描一天的范围。那么我们就可以改写SQL啦!

mysql > select * from testtable WHERE biz_date >= '2017-08-20 00:00:00' and biz_date <= '2017-08-21 00:00:00' and status = 2;
Empty set (0.00 sec)

mysql > desc select * from testtable WHERE biz_date >= '2017-08-20 00:00:00' and biz_date <= '2017-08-21 00:00:00' and status = 2;
+----+-------------+------------------+-------+----------------+----------------+---------+------+------+-----------------------+
| id | select_type | table            | type  | possible_keys  | key            | key_len | ref  | rows | Extra                 |
+----+-------------+------------------+-------+----------------+----------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | testtable        | range | idx_bizdate_st | idx_bizdate_st | 6       | NULL |  789 | Using index condition |
+----+-------------+------------------+-------+----------------+----------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

-- rows降低了很多,乖乖的走了索引

mysql > desc select * from testtable WHERE biz_date >= '2017-08-20 00:00:00' and biz_date <= '2017-08-21 00:00:00' ;
+----+-------------+------------------+-------+----------------+----------------+---------+------+------+-----------------------+
| id | select_type | table            | type  | possible_keys  | key            | key_len | ref  | rows | Extra                 |
+----+-------------+------------------+-------+----------------+----------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | testtable        | range | idx_bizdate_st | idx_bizdate_st | 5       | NULL | 1318 | Using index condition |
+----+-------------+------------------+-------+----------------+----------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

-- 即使没有status,也是肯定走索引啦

小结

这个问题,我原本打算用hint,强制让他走索引,但是实际上强制走索引的执行时间并不能带来满意的效果。结合业务逻辑,来优化SQL,是最好的方式,也是终极法宝,一定要好好利用。不了解业务的DBA,不是一个好DBA...