MySQL索引最左前缀原则导致系统瘫痪

早上九点半左右 业务人员反映他们页面打开缓慢，后续页面出现502。

然后我这边收到报警 ，登录数据库服务器（4核cpu）查看 cpu 400% load 30左右

进入到数据库中查看发现好多慢查询

本以为这些慢查询是来自该系统每天的定时任务（该系统相当于一个olap系统，每天会进行批量的数据查询提取。）于是先crontab -e 把所有的定时任务都停掉。但是慢查询还是存在。

所有的慢查询都是同一个模板， 后来询问开发的同事昨天上线了新版本

SELECT (plan.due_amount + IF(plan.overdue_day > 0, (plan.due_amount * extend.supplement_penalty_rate /100), 0) - plan.reduce_amount) due_amount_total, plan.repay_status                     FROM mostop_xiaodai_supplement_loan_repay_plan plan LEFT JOIN mostop_xiaodai_loan_info_extend extend ON extend.loan_id = plan.loan_id WHERE plan.base_plan_id = 11124546 AND plan.step_no = 2

查看执行表结构

mysql> show create table mostop_xiaodai_loan_info_extend\G*************************** 1. row ***************************       Table: mostop_xiaodai_loan_info_extendCreate Table: CREATE TABLE `mostop_xiaodai_loan_info_extend` (  `id` bigint(20) unsigned NOT NULL COMMENT '编号',  `agentid` int(10) unsigned NOT NULL COMMENT '渠道 ID',  `loan_id` bigint(20) unsigned NOT NULL COMMENT '贷款编号',  `create_time` datetime NOT NULL COMMENT '创建时间',  `update_time` datetime NOT NULL COMMENT '更新时间',  `total_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '总利率',  `service_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '服务费率',  `intrest_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '利息费率',  `overdue_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '逾期总利率',  `overdue_service_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '逾期服务费率',  `penalty_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '违约金率',  `is_split` tinyint(4) DEFAULT '0' COMMENT '息费打平，是否需要拆单',  `desired_repay_type` varchar(9) DEFAULT NULL COMMENT '息费打平，理想还款方式',  `desired_total_rate` decimal(10,6) DEFAULT NULL COMMENT '息费打平，理想总利率',  `supplement_overdue_rate` decimal(10,6) DEFAULT NULL COMMENT '息费打平，白条订单逾期总利率',  `supplement_penalty_rate` decimal(10,6) DEFAULT NULL COMMENT '息费打平，白条订单违约金率',  `investor_rate` decimal(10,6) DEFAULT NULL COMMENT '投资人利率',  `investor_repay_type` varchar(9) DEFAULT NULL COMMENT '投资人利率',  PRIMARY KEY (`id`,`agentid`),  UNIQUE KEY `agentid` (`agentid`,`loan_id`)  ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='贷款信息扩展表'

查看执行计划

mysql> explain SELECT (plan.due_amount + IF(plan.overdue_day > 0, (plan.due_amount * extend.supplement_penalty_rate /100), 0) | id | select_type | table  | partitions | type  | possible_keys   | key             | key_len | ref   | rows    | filtered | Extra       |+----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+---------+----------+-------------+|  1 | SIMPLE      | plan   | NULL       | const | idx_base_planid | idx_base_planid | 8       | const |       1 |   100.00 | NULL        ||  1 | SIMPLE      | extend | NULL       | ALL   | NULL            | NULL            | NULL    | NULL  | 4690305 |   100.00 | Using where |+----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+---------+----------+-------------+

虽然表中的联合索引上有loan_id这个列

 UNIQUE KEY `agentid` (`agentid`,`loan_id`)

但是根据索引的最左前缀原则，where条件中直接出了loan_id，复合索引出现了断开，所以索引失效。研发同学以为是可以用到表中的索引，没有审核就上线了，所以导致了全表扫描导致服务器的负载超高。

解决办法

添加索引

alter table mostop_xiaodai_loan_info_extend add index IDX_loan_id (loan_id);

添加索引后执行计划

explain SELECT (plan.due_amount + IF(plan.overdue_day > 0, (plan.due_amount * extend.supplement_penalty_rate /100), 0) - plan.reduce_amount) due_amount_total, plan.repay_status    ->                      FROM mostop_xiaodai_supplement_loan_repay_plan plan LEFT JOIN mostop_xiaodai_loan_info_extend extend ON extend.loan_id = plan.loan_id WHERE plan.base_plan_id = 11124546 AND plan.step_no = 2 ;+----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+------+----------+-------------+| id | select_type | table  | partitions | type  | possible_keys   | key             | key_len | ref   | rows | filtered | Extra       |+----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+------+----------+-------------+|  1 | SIMPLE      | plan   | NULL       | const | idx_base_planid | idx_base_planid | 8       | const |    1 |   100.00 | NULL        ||  1 | SIMPLE      | extend | NULL       | ref   | IDX_loan_id     | IDX_loan_id     | 8       | const |    1 |   100.00 | Using where |+----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+------+----------+-------------+

服务器负载立马回复正常

通过本次事故

上线前进行SQL审核

应用和数据库单独部署在不同服务器上