MySQL索引最左字首原則導致系統癱瘓
阿新 • • 發佈:2018-12-28
早上九點半左右 業務人員反映他們頁面開啟緩慢,後續頁面出現502。
然後我這邊收到報警 ,登入資料庫伺服器(4核cpu)檢視 cpu 400% load 30左右
進入到資料庫中檢視發現好多慢查詢
本以為這些慢查詢是來自該系統每天的定時任務(該系統相當於一個olap系統,每天會進行批量的資料查詢提取。)於是先crontab -e 把所有的定時任務都停掉。但是慢查詢還是存在。
所有的慢查詢都是同一個模板, 後來詢問開發的同事昨天上線了新版本
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
檢視執行表結構
mysql> show create table mostop_xiaodai_loan_info_extend\G *************************** 1. row *************************** Table: mostop_xiaodai_loan_info_extend Create Table: CREATE TABLE `mostop_xiaodai_loan_info_extend` ( `id` bigint(20) unsigned NOT NULL COMMENT '編號', `agentid` int(10) unsigned NOT NULL COMMENT '渠道 ID', `loan_id` bigint(20) unsigned NOT NULL COMMENT '貸款編號', `create_time` datetime NOT NULL COMMENT '建立時間', `update_time` datetime NOT NULL COMMENT '更新時間', `total_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '總利率', `service_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '服務費率', `intrest_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '利息費率', `overdue_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '逾期總利率', `overdue_service_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '逾期服務費率', `penalty_rate` decimal(10,6) unsigned DEFAULT NULL COMMENT '違約金率', `is_split` tinyint(4) DEFAULT '0' COMMENT '息費打平,是否需要拆單', `desired_repay_type` varchar(9) DEFAULT NULL COMMENT '息費打平,理想還款方式', `desired_total_rate` decimal(10,6) DEFAULT NULL COMMENT '息費打平,理想總利率', `supplement_overdue_rate` decimal(10,6) DEFAULT NULL COMMENT '息費打平,白條訂單逾期總利率', `supplement_penalty_rate` decimal(10,6) DEFAULT NULL COMMENT '息費打平,白條訂單違約金率', `investor_rate` decimal(10,6) DEFAULT NULL COMMENT '投資人利率', `investor_repay_type` varchar(9) DEFAULT NULL COMMENT '投資人利率', PRIMARY KEY (`id`,`agentid`), UNIQUE KEY `agentid` (`agentid`,`loan_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='貸款資訊擴充套件表'
檢視執行計劃
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 | +----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+---------+----------+-------------+
雖然表中的聯合索引上有loan_id這個列
UNIQUE KEY `agentid` (`agentid`,`loan_id`)
但是根據索引的最左字首原則,where條件中直接出了loan_id,複合索引出現了斷開,所以索引失效。研發同學以為是可以用到表中的索引,沒有稽核就上線了,所以導致了全表掃描導致伺服器的負載超高。
解決辦法
新增索引
alter table mostop_xiaodai_loan_info_extend add index IDX_loan_id (loan_id);
新增索引後執行計劃
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 |
+----+-------------+--------+------------+-------+-----------------+-----------------+---------+-------+------+----------+-------------+
伺服器負載立馬恢復正常
通過本次事故
上線前進行SQL稽核
應用和資料庫部署在不同伺服器上