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

 

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