由delete引起的锁扩大
阿里云月报中的一句话，出处：http://mysql.taobao.org/monthly/2022/01/01/
但是Ghost Record是可以跟正常的Record一样作为Key Range Lock的加锁对象的。可以看出这相当于把删除操作变成了更新操作，因此删除事务不在需要持有Next Key Lock
这句话意思是：假设delete语句物理删除数据，那么delete事务会持有gap lock，那么会造成锁扩大，而实际上delete操作会转为update操作，最终delete事务持有的gap lock退化为record lock，不会造成锁扩大
下面用SQL Server和MySQL做测试，看一下锁的情况
SQL Server 2012
use test go
CREATE TABLE t ( id int NOT NULL primary key, c int DEFAULT NULL, d int DEFAULT NULL ) CREATE NONCLUSTERED INDEX [ix_t_c] ON [dbo].[t] ( [c] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] GO insert into t values(5,5,5),(10,10,10),(20,20,20),(25,25,25);
使用下面的执行顺序
在session1执行下面语句
--session 1 USE test GO SET TRANSACTION ISOLATION LEVEL SERIALIZABLE GO begin transaction select id from t where c >10 and c <= 24 delete from t where c = 25 --commit
在session2执行下面语句
--session 2 USE test GO SET TRANSACTION ISOLATION LEVEL SERIALIZABLE GO insert into t(id,c,d) values(27,27,27); (blocked)
申请的锁如下
分析：首先我们要关注的加锁对象是二级索引【ix_t_c】，可以看到有三个range锁，这里锁住的范围是
rangeS-S(10,20]
rangeX-X(20, 25]
rangeS-U[25, +∞) 正无穷
正因为rangeS-U 锁，session 2的insert操作被阻塞了，也就是删除 c=25 这行数据，导致键范围锁扩大到 正无穷
MySQL 8.0.28
set global transaction isolation level REPEATABLE READ; select @@global.transaction_isolation; use test; CREATE TABLE `t` ( `id` int(11) NOT NULL, `c` int(11) DEFAULT NULL, `d` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `c` (`c`) ) ENGINE=InnoDB; insert into t values(5,5,5),(10,10,10),(20,20,20),(25,25,25);
SQL语句执行顺序跟SQL Server一样
在session1执行下面语句
-- session 1 begin; select id from t where c >10 and c <= 24 for update; delete from t where c = 25; --commit
在session2执行下面语句
-- session 2 insert into t(id,c,d) values(27,27,27); （blocked）
申请的锁如下
select * from performance_schema.data_locks\G *************************** 1. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552409600:1217:140111564061632 ENGINE_TRANSACTION_ID: 7643 THREAD_ID: 331 EVENT_ID: 8 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: NULL OBJECT_INSTANCE_BEGIN: 140111564061632 LOCK_TYPE: TABLE LOCK_MODE: IX LOCK_STATUS: GRANTED LOCK_DATA: NULL *************************** 2. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552409600:59:5:1:140111564058528 ENGINE_TRANSACTION_ID: 7643 THREAD_ID: 331 EVENT_ID: 8 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: c OBJECT_INSTANCE_BEGIN: 140111564058528 LOCK_TYPE: RECORD LOCK_MODE: X,INSERT_INTENTION LOCK_STATUS: WAITING LOCK_DATA: supremum pseudo-record *************************** 3. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552408792:1217:140111564055552 ENGINE_TRANSACTION_ID: 7642 THREAD_ID: 330 EVENT_ID: 12 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: NULL OBJECT_INSTANCE_BEGIN: 140111564055552 LOCK_TYPE: TABLE LOCK_MODE: IX LOCK_STATUS: GRANTED LOCK_DATA: NULL *************************** 4. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552408792:59:5:1:140111564052496 ENGINE_TRANSACTION_ID: 7642 THREAD_ID: 330 EVENT_ID: 12 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: c OBJECT_INSTANCE_BEGIN: 140111564052496 LOCK_TYPE: RECORD LOCK_MODE: X LOCK_STATUS: GRANTED LOCK_DATA: supremum pseudo-record *************************** 5. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552408792:59:5:4:140111564052496 ENGINE_TRANSACTION_ID: 7642 THREAD_ID: 330 EVENT_ID: 12 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: c OBJECT_INSTANCE_BEGIN: 140111564052496 LOCK_TYPE: RECORD LOCK_MODE: X LOCK_STATUS: GRANTED LOCK_DATA: 20, 20 *************************** 6. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552408792:59:5:5:140111564052496 ENGINE_TRANSACTION_ID: 7642 THREAD_ID: 330 EVENT_ID: 12 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: c OBJECT_INSTANCE_BEGIN: 140111564052496 LOCK_TYPE: RECORD LOCK_MODE: X LOCK_STATUS: GRANTED LOCK_DATA: 25, 25 *************************** 7. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552408792:59:4:4:140111564052840 ENGINE_TRANSACTION_ID: 7642 THREAD_ID: 330 EVENT_ID: 12 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: PRIMARY OBJECT_INSTANCE_BEGIN: 140111564052840 LOCK_TYPE: RECORD LOCK_MODE: X,REC_NOT_GAP LOCK_STATUS: GRANTED LOCK_DATA: 20 *************************** 8. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 140111552408792:59:4:5:140111564052840 ENGINE_TRANSACTION_ID: 7642 THREAD_ID: 330 EVENT_ID: 12 OBJECT_SCHEMA: test OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: PRIMARY OBJECT_INSTANCE_BEGIN: 140111564052840 LOCK_TYPE: RECORD LOCK_MODE: X,REC_NOT_GAP LOCK_STATUS: GRANTED LOCK_DATA: 25 8 rows in set (0.00 sec)
分析：这里我们要关注的加锁对象依然是二级索引【c】，这里跟SQL Server一样
LOCK_MODE: X
LOCK_STATUS: GRANTED
LOCK_DATA: supremum pseudo-record
锁住的范围是 [25, +∞) 正无穷， 所以session 2的insert操作被阻塞了，也就是删除 c=25 这行数据，导致gap lock 扩大到 正无穷
通过上面两个测试，可以知道即使delete操作留下了Ghost Records，但是delete事务造成的gap lock没缩小为Ghost Record的 record lock
因此，阿里云月报中的说法有失偏颇
本文版权归作者所有，未经作者同意不得转载。