MySQL Index--关联条件列索引缺失导致执行计划性能不佳

某系统反馈慢SQL影响生产，查看SLOW LOG发现下面慢SQL:

SELECT COUNT(DISTINCT m.batch_no)
FROM ob_relation r
INNER JOIN ob_batch_d d
ON r.sub_order_no = d.outbound_no
INNER JOIN ob_batch_m m
ON d.batch_no = m.batch_no
WHERE r.production_mode =1
AND r.yn=0
AND r.outbound_no ='xxxx2156'
AND d.yn=0
AND m.yn=0
AND m.SEND_FLAG=0
AND m.batch_no!='xxx00025984';

设计表上索引情况如下：

表ob_relation上索引：
KEY `idx_outbound_no` (`OUTBOUND_NO`),
KEY `idx_sub_order_no` (`SUB_ORDER_NO`)

表ob_batch_d上索引：
KEY `idx_update_time` (`UPDATE_TIME`),
KEY `idx_ob_outbound_batch_d_batch_no` (`BATCH_NO`),
KEY `idx_ORDER_GROUP_ID` (`ORDER_GROUP_ID`),

表ob_batch_m上索引：
KEY `idx_update_time` (`UPDATE_TIME`),
KEY `idx_BATCH_NO` (`BATCH_NO`),
KEY `idx_BEAT_NO` (`BEAT_NO`),
KEY `idx_BATCH_TYPE_OPT_STATUS` (`BATCH_TYPE`,`OPT_STATUS`),
KEY `ix_SEND_FLAG` (`SEND_FLAG`)

查看执行计划为：

+----+-------------+-------+------------+------+----------------------------------+----------------------------------+---------+--------------------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys                    | key                              | key_len | ref                | rows  | filtered | Extra       |
+----+-------------+-------+------------+------+----------------------------------+----------------------------------+---------+--------------------+-------+----------+-------------+
|  1 | SIMPLE      | r     | NULL       | ref  | idx_outbound_no,idx_sub_order_no | idx_outbound_no                  | 92      | const              |     4 |     1.25 | Using where |
|  1 | SIMPLE      | m     | NULL       | ref  | idx_BATCH_NO,ix_SEND_FLAG        | ix_SEND_FLAG                     | 2       | const              | 54292 |     5.07 | Using where |
|  1 | SIMPLE      | d     | NULL       | ref  | idx_ob_outbound_batch_d_batch_no | idx_ob_outbound_batch_d_batch_no | 92      | ob_task.m.BATCH_NO |    16 |     1.00 | Using where |
+----+-------------+-------+------------+------+----------------------------------+----------------------------------+---------+--------------------+-------+----------+-------------+

本着直觉判定使用索引ix_SEND_FLAG但影响行数为54292步骤存在问题，虽然过滤条件中包含SEND_FLAG=0且列SEND_FLAG上有索引，但选择性较差，初步断定查询走错索引导致。

尝试1：删除列SEND_FLAG上索引，迫使查询走列BATCH_NO上索引，删除列SEND_FLAG上索引后，执行计划变为：

+----+-------------+-------+------------+-------+----------------------------------+----------------------------------+---------+--------------------+--------+----------+---------------------------------------------------------------------------+
| id | select_type | table | partitions | type  | possible_keys                    | key                              | key_len | ref                | rows   | filtered | Extra                                                                     |
+----+-------------+-------+------------+-------+----------------------------------+----------------------------------+---------+--------------------+--------+----------+---------------------------------------------------------------------------+
|  1 | SIMPLE      | r     | NULL       | ref   | idx_outbound_no,idx_sub_order_no | idx_outbound_no                  | 92      | const              |      4 |     1.25 | Using where                                                               |
|  1 | SIMPLE      | m     | NULL       | range | idx_BATCH_NO                     | idx_BATCH_NO                     | 92      | NULL               | 146187 |     9.99 | Using index condition; Using where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | d     | NULL       | ref   | idx_ob_outbound_batch_d_batch_no | idx_ob_outbound_batch_d_batch_no | 92      | ob_task.m.BATCH_NO |     16 |     1.00 | Using where                                                               |
+----+-------------+-------+------------+-------+----------------------------------+----------------------------------+---------+--------------------+--------+----------+---------------------------------------------------------------------------+

查询影响行数变得更高，性能更差。

尝试2：检查INNER JOIN 关联列和WHERE条件列上的数据类型，排除隐式转换导致。

尝试3： 去除与ob_batch_m的所有相关进行，仅剩表ob_relation和ob_batch_d做INNER JOIN，其执行计划为：

SELECT COUNT(DISTINCT d.batch_no)
FROM ob_relation r
INNER JOIN ob_batch_d d
ON r.sub_order_no = d.outbound_no
WHERE r.production_mode =1
AND r.yn=0
AND r.outbound_no ='xxxx2156'
AND d.yn=0 

+----+-------------+-------+------------+------+----------------------------------+-----------------+---------+-------+---------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys                    | key             | key_len | ref   | rows    | filtered | Extra                                              |
+----+-------------+-------+------------+------+----------------------------------+-----------------+---------+-------+---------+----------+----------------------------------------------------+
|  1 | SIMPLE      | r     | NULL       | ref  | idx_outbound_no,idx_sub_order_no | idx_outbound_no | 92      | const |       4 |     1.25 | Using where                                        |
|  1 | SIMPLE      | d     | NULL       | ALL  | NULL                             | NULL            | NULL    | NULL  | 4917114 |     1.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+----------------------------------+-----------------+---------+-------+---------+----------+----------------------------------------------------+

发现表ob_batch_d的关联条件上缺少索引导致。

问题总结：

查询中ob_relation和ob_batch_d关联，而ob_batch_d和ob_batch_m关联，但由于表ob_batch_d关联条件上没有索引且表ob_relation上WHERE过滤条件选择性极高，因此查询优化器将SQL转换为：

SELECT COUNT(DISTINCT m1.batch_no) FROM (
	SELECT m.batch_no,d.outbound_no
	FROM ob_batch_d d
	INNER JOIN ob_batch_m m
	ON d.batch_no = m.batch_no
	WHERE d.yn=0
	AND m.yn=0
	AND m.SEND_FLAG=0
	AND m.batch_no!='xxx00025984'
)AS m1
INNER JOIN (
	SELECT r.sub_order_no
	FROM ob_relation r
	WHERE r.production_mode =1
	AND r.yn=0
	AND r.outbound_no ='xxxx2156'
)AS r1
ON m1.outbound_no=r1.sub_order_no

对于m1子查询内部，表ob_batch_m可以通过SEND_FLAG=0来过滤部分数据(虽然选择性较差)，因此选择使用索引ix_SEND_FLAG，而表ob_batch_d上缺少索引的问题被“完美隐藏”。

在优化INNER JOIN操作时，通常需要先确定两表的关系（外表和内表），外表通过WHERE条件列来过滤数据，而内表通过关联条件列来定位数据。

如对于下列查询:

SELECT COUNT(1)
FROM ob_relation r
INNER JOIN ob_batch_d d
ON r.sub_order_no = d.outbound_no
WHERE r.outbound_no ='xxxx2156';

表ob_relation和表ob_batch_d做INNER JOIN操作
1、表ob_relation作为外表，可以通过outbound_no ='xxxx2156'来过滤数据，因此需要在ob_relation(outbound_no)上创建索引。

2、表ob_batch_d作为内表，需要通过关联列outbound_no来定位数据，因此需要在ob_batch_d(outbound_no)上创建索引。

3、表ob_relation的关联条件sub_order_no并不需要单独作为索引第一列来创建索引，但在部分查中可以通过覆盖索引特性来优化查询，如上面查询中创建组合索引ob_relation(outbound_no,sub_order_no)能获得最佳性能。

总结：

有时候，经验能帮助你快速处理问题，但有些时候，经验能带你直接走入误区。