GreatSQL 中 Insert 慢是什么情况？

* GreatSQL社区原创内容未经授权不得随意使用，转载请联系小编并注明来源。 背景概述 客户反映，业务上某张表的 insert 操作速度很慢，单条 insert 语句的最大执行时间超过了 5 秒。在收到客户问题后，我们仔细检查了数据库状态以及主机的负载情况，发现目前一切正常，并没有发现数据库故障或主机负载过高导致 insert 操作变慢的问题。 因此，我们分析了慢日志，希望从中找出问题。经过分析，发现这条插入语句的query_time和lock_time几乎相同，因此怀疑是由于锁等待导致插入操作变慢。随后，我们捕获了通用日志，几乎同一时间这张表有update，insert操作，发现由于更新操作阻塞了插入操作，导致插入速度下降的问题。这个更新操作所在的事务包含了多条 SQL 语句，因此如果该事务执行时间较长，就会阻塞插入操作，导致插入操作的执行时间延长。 问题复现 本次测试基于 GreatSQL 8.0.32-25，隔离级别为 RR 2.1 创建测试表 greatsql> CREATE TABLE `t11` ( `id` int NOT NULL, `c1` int DEFAULT NULL, `c2` int DEFAULT NULL, `c3` int DEFAULT NULL, `c4` int DEFAULT NULL, PRIMARY KEY (`id`), KEY `c2` (`c2`,`c3`), KEY `c4` (`c4`) ); greatsql> insert into t11 values (1,1,1,1,1),(2,2,2,2,2),(3,3,3,3,3),(5,5,5,5,5); 2.2 事务执行顺序 时间事务1事务2T1BEGIN;BEGIN;T2update t10 set c2=20 where c4=2; T3 insert into t10 values (6,2,2,2,2);T4 -- hang住，处于锁等待T5commit;-- 锁等待结束T6 commit; 2.3 事务1执行 greatsql> begin; greatsql> update t11 set c2=20 where c4=2; 查看加锁情况： greatsql> select THREAD_ID,EVENT_ID,ENGINE_LOCK_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+----------+-------------------------------------------+---------------+-------------+------------+-----------+---------------+-------------+-----------+ | THREAD_ID | EVENT_ID | ENGINE_LOCK_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+----------+-------------------------------------------+---------------+-------------+------------+-----------+---------------+-------------+-----------+ | 55 | 20 | 140531661278568:44172:140531678523168 | test | t11 | NULL | TABLE | IX | GRANTED | NULL | | 55 | 20 | 140531661278568:43110:6:3:140531678129184 | test | t11 | c4 | RECORD | X | GRANTED | 2, 2 | | 55 | 20 | 140531661278568:43110:4:3:140531678129528 | test | t11 | PRIMARY | RECORD | X,REC_NOT_GAP | GRANTED | 2 | | 55 | 20 | 140531661278568:43110:6:4:140531678129872 | test | t11 | c4 | RECORD | X,GAP | GRANTED | 3, 3 | +-----------+----------+-------------------------------------------+---------------+-------------+------------+-----------+---------------+-------------+-----------+ 4 rows in set (0.01 sec) 可以看到此时给【3, 3】这条数据加加了X,GAP锁 2.4 事务2执行 greatsql> begin; greatsql> insert into t11 values (6,2,2,2,2); 查看加锁情况： greatsql> select THREAD_ID,EVENT_ID,ENGINE_LOCK_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+----------+-------------------------------------------+---------------+-------------+------------+-----------+------------------------+-------------+-----------+ | THREAD_ID | EVENT_ID | ENGINE_LOCK_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+----------+-------------------------------------------+---------------+-------------+------------+-----------+------------------------+-------------+-----------+ | 56 | 14 | 140531661279416:44172:140531678523936 | test | t11 | NULL | TABLE | IX | GRANTED | NULL | | 56 | 14 | 140531661279416:43110:6:4:140531678132256 | test | t11 | c4 | RECORD | X,GAP,INSERT_INTENTION | WAITING | 3, 3 | | 55 | 20 | 140531661278568:44172:140531678523168 | test | t11 | NULL | TABLE | IX | GRANTED | NULL | | 55 | 20 | 140531661278568:43110:6:3:140531678129184 | test | t11 | c4 | RECORD | X | GRANTED | 2, 2 | | 55 | 20 | 140531661278568:43110:4:3:140531678129528 | test | t11 | PRIMARY | RECORD | X,REC_NOT_GAP | GRANTED | 2 | | 55 | 20 | 140531661278568:43110:6:4:140531678129872 | test | t11 | c4 | RECORD | X,GAP | GRANTED | 3, 3 | +-----------+----------+-------------------------------------------+---------------+-------------+------------+-----------+------------------------+-------------+-----------+ 6 rows in set (0.00 sec) greatsql> select REQUESTING_THREAD_ID,REQUESTING_EVENT_ID,REQUESTING_ENGINE_LOCK_ID,BLOCKING_THREAD_ID,BLOCKING_EVENT_ID,BLOCKING_ENGINE_LOCK_ID from performance_schema.data_lock_waits; +----------------------+---------------------+-------------------------------------------+--------------------+-------------------+-------------------------------------------+ | REQUESTING_THREAD_ID | REQUESTING_EVENT_ID | REQUESTING_ENGINE_LOCK_ID | BLOCKING_THREAD_ID | BLOCKING_EVENT_ID | BLOCKING_ENGINE_LOCK_ID | +----------------------+---------------------+-------------------------------------------+--------------------+-------------------+-------------------------------------------+ | 56 | 14 | 140531661279416:43110:6:4:140531678132256 | 55 | 20 | 140531661278568:43110:6:4:140531678129872 | +----------------------+---------------------+-------------------------------------------+--------------------+-------------------+-------------------------------------------+ 1 row in set (0.00 sec) 通过上面2张表，可以看到 X,GAP锁 阻塞了 X,GAP,INSERT_INTENTION 锁； 2.5 结论 此次Insert慢的原因就是Update语句所在的事务执行时间较长，Update语句产生了GAP锁； Insert语句在执行时此Update语句所在事务还没有执行完成，因此Insert处于锁等待阶段，待Update所在事务提交后Insert才提交； 总结 导致此次问题的原因是 GAP 锁阻塞了 INSERT_INTENTION 锁；因此建议客户在执行 update 操作时，where 条件用主键列，这样可以避免加 GAP 锁。