收到运维同学电话，MySQL服务器连接数满了，登录服务器查看，确实满了，好吧，首先增加连接数到2500，暂时提供对外服务。连接继续升高，又快达到2500。发现有大量的查询时间将近到了1200秒，大量的长连接堆积，导致连接数攀升，看来还是sql的问题。在这些长连接中，发现这样的sql

SELECT product_id,gift_id,gift_original_price,gift_count, FROM promo_xxx WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1;

mysql> explain SELECT product_id,gift_id,gift_original_price,gift_count FROM promo_gift WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1;

+----+-------------+-----------------+------+---------------+------+---------+------+--------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-----------------+------+---------------+------+---------+------+--------+-------------+

| 1 | SIMPLE | promo_gift_list | ALL | id_promo_gift | NULL | NULL | NULL | 249188 | Using where |

+----+-------------+-----------------+------+---------------+------+---------+------+--------+-------------+

1 row in set (0.04 sec)

mysql> show index from promo_gift;

+-----------------+------------+-----------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |

+-----------------+------------+-----------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| promo_gift_list | 0 | PRIMARY | 1 | id | A | 261184 | NULL | NULL | | BTREE | | |

| promo_gift_list | 0 | id_promo_gift | 1 | promotion_id | A | 1140 | NULL | NULL | YES | BTREE | | |

| promo_gift_list | 0 | id_promo_gift | 4 | product_id | A | 261184 | NULL | NULL | YES | BTREE | | |

狗血的sql，竟然走全表扫描，但是promotion_id有索引啊，为什么没有走索引呢？而且以前建立的索引，走的好好的，今天怎么就出现问题了，这是一个问题

那我们可以通过last_query_cost 查看sql消耗

mysql>SELECT product_id,gift_id,gift_original_price,gift_count FROM promo_gift WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1;

mysql>show status like 'last_query_cost';

+-----------------+--------------+

| Variable_name | Value |

+-----------------+--------------+

| Last_query_cost | 52626.599000 |

+-----------------+--------------+

1 row in set (0.00 sec)

不走索引，那我们强制使用索引

mysql> explain SELECT product_id,gift_id,gift_original_price,gift_count FROM promo_gift force index(id_promo_gift) WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1;

+----+-------------+-----------------+-------+---------------+---------------+---------+------+--------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-----------------+-------+---------------+---------------+---------+------+--------+-------------+

| 1 | SIMPLE | promo_gift_list | range | id_promo_gift | id_promo_gift | 5 | NULL | 124594 | Using where |

+----+-------------+-----------------+-------+---------------+---------------+---------+------+--------+-------------+

1 row in set (0.02 sec)

嗯，加上索引了，那么sql消耗怎么样呢？

mysql> SELECT product_id,gift_id,gift_original_price,gift_count FROM promo_gift force index(id_promo_gift) WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1;

+------------+----------+---------------------+------------+

| product_id | gift_id | gift_original_price | gift_count |

+------------+----------+---------------------+------------+

| 22569455 | 23230046 | 147.00 | 1 |

+------------+----------+---------------------+------------+

1 row in set (0.40 sec)

mysql> show status like 'last_query_cost';

+-----------------+---------------+

| Variable_name | Value |

+-----------------+---------------+

| Last_query_cost | 174432.609000 |

+-----------------+---------------+

1 row in set (0.00 sec)

我们发现如果使用这个索引，sql消耗174432.609000>52626.599000,mysql优化器认为使用这个id_promo_gift索引，sql消耗是非常大的，这就是mysql执行不使用这个索引的原因。

后来开发人员说，昨天晚上这个表增加了11万多的数据，嗯，数据量增加，mysql执行计划改变。那好吧，单独product_id列再加一个索引。

mysql> alter table promo_gift_list add index product_id(product_id);

Query OK, 0 rows affected (6.45 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> explain SELECT product_id,gift_id,gift_original_price,gift_count FROM promo_gift_list WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1 AND IFNULL(is_delete,0)!=1;

+----+-------------+-----------------+------+--------------------------+------------+---------+-------+------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-----------------+------+--------------------------+------------+---------+-------+------+-------------+

| 1 | SIMPLE | promo_gift_list | ref | id_promo_gift,product_id | product_id | 5 | const | 2 | Using where |

+----+-------------+-----------------+------+--------------------------+------------+---------+-------+------+-------------+

1 row in set (0.00 sec)

使用了刚才新加的索引 product_id

mysql> SELECT product_id,gift_id,gift_original_price,gift_count FROM promo_gift_list WHERE promotion_id IN (589994,589994) AND product_id IN (22569455) AND is_valid=1 AND IFNULL(is_delete,0)!=1;

+------------+----------+---------------------+------------+

| product_id | gift_id | gift_original_price | gift_count |

+------------+----------+---------------------+------------+

| 22569455 | 23230046 | 147.00 | 1 |

+------------+----------+---------------------+------------+

1 row in set (0.00 sec)

mysql> show status like 'last_query_cost';

+-----------------+----------+

| Variable_name | Value |

+-----------------+----------+

| Last_query_cost | 2.399000 |

+-----------------+----------+

1 row in set (0.01 sec)

sql消耗降到了2.399000，ok，问题解决。连接数很快从1000多降到100以内。

数据量的增加导致了mysql执行计划的改变，那么mysql的cost是怎么计算的呢？

cost=io_cost+cpu_cost

cpu_cost位于mysql上层，处理返回的记录所花开销，io_cost存储引擎层，读取也没的IO开销。最直接的方式last_query_cost记录sql的cost。查看last_query_cost可以初步判断sql的cost，明白mysql优化器执行的依据。
参考网址见:https://blog.csdn.net/weixin_35980267/article/details/113223849