一、前言

开发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，已经在相应的查询条件上加上了索引

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

二、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。如果有疑问，请参考文章 10分钟让你明白MySQL是如何利用索引的

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 whwtestbuffer;
+------------+
| biz_status |
+------------+
|          1 |
|          2 |
|          4 |
+------------+

通过以上查询，我们可以发现如下几点问题：

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？
是的，相对于status字段，biz_date 的选择性更加不错，那组合索引本身已经没有好调整了

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

三、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… 继续去补业务知识去了。。