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由delete语句引起的锁范围扩大
由delete语句引起的锁范围扩大
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，因此删除事务不再需要持有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，不会造成锁范围扩大
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，情况如下
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，情况如下
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MySQL的情况跟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操作在数据表中留下了Ghost Records，但是delete事务造成的gap lock范围没有缩小为Ghost Record的 record lock
因此，阿里云内核月报中的说法有失偏颇，误导读者