传送门：MySQL锁：01.总览
传送门：MySQL锁：02.InnoDB锁
传送门：MySQL锁：03.InnoDB行锁
通过索引实现行锁，在索引记录上加锁。
没有索引就无法实现行锁，升级成全表记录锁，等同于表锁。
理解InnoDB独特的行锁运行机制，认识特有的四种行锁粒度——lock_ordinary、lock_gap、lock_rec_not_gap、lock_insert_intention
InnoDB 行锁
- 默认都是加lock_ordinary锁
- 如果是唯一索引列上的等值查询，则退化成lock_rec_not_gap
- 所有版本，非唯一索引列上的范围查询，遇到第一个不符合条件的记录也会加上lock_ordinary。
- 8.0.18版本以前，主要指<=场景：唯一索引列上的范围查询，遇到第一个不符合条件的记录也会加上lock_ordinary ，在RC下会释放，RR下不会释放。
- 8.0.18版本以前，非唯一索引列上的等值查询，向右遍历遇到第一个不符合条件的记录时，先加上lock_ordinary，再退化成lock_gap。
锁排查可以用的视图和数据字典
mysql> show engine innodb status G
mysql> select * from performance_schema.data_lock_waits;
mysql> select * from performance_schema.data_locks;
mysql> select * from performance_schema.metadata_locks;
InnoDB 行锁兼容性
| 请求的锁类型 | 请求的锁类型 | 请求的锁类型 | 请求的锁类型 | ||
|---|---|---|---|---|---|
| lock_ordinary | lock_rec_not_gap | lock_gap | lock_insert_intention | ||
| 已获得的锁类型 | lock_ordinary | X | X | O | X |
| 已获得的锁类型 | lock_rec_not_gap | X | X | O | O |
| 已获得的锁类型 | lock_gap | O | O | O | X |
| 已获得的锁类型 | lock_insert_intention | O | O | O | O |
-
gap只和insert intention锁冲突
-
insert intention和任何锁都不冲突，除非也在相同位置做意向插入锁
-
先获得意向插入锁的，再尝试上gap lock是可以的
-
但是反过来 ，先获得gap lock的，再尝试加上意向插入锁便会阻塞，
-
原因是：先获得意向插入锁时，实际上插入已经成功，意向插入锁会被转变为对具体记录的ordinary 或 rec_not_gap ，此时二者都与lock gap兼容。
InnoDB行锁之共享锁
共享锁：
- 不允许其他事务修改被锁定的行，只能读
- select .. for share/ lock in share mode
- 自动提交模式下的普通select是一致性非锁定读，不加锁。
自动提交模式下， 不使用begin开启事务，直接select的话：
select * from xxx where .. 不加锁
select * from xxx where .. for share ，也查询不到加锁， 但是实际上是加锁的，只不过锁的时间及其的短暂。
验证：
此时，用排他锁来验证自动提交模式的for share究竟是否产生锁动作。
可以看出，自动提交模式下select（不加for share）是一致性非锁定读，但是加for share后，是会有锁定动作的，只不过没有阻塞的情况下，锁的持续时间是非常短暂的。
查看InnoDB锁
-
开启参数：innodb_status_output_locks=1; 以支持使用 show engine innodb status 查看锁详情。
mysql> begin ; select * from k1 where id=4 for update; Query OK, 0 rows affected (0.00 sec) mysql> show engine innodb status G .. MySQL thread id 31, OS thread handle 139620328457984, query id 1297 localhost root **TABLE LOCK table `kk`.`k1` trx id 1901 lock mode IX** RECORD LOCKS space id 3 page no 10 n bits 1056 index id of table `kk`.`k1` trx id 1901 lock_mode X Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000004; asc ;; 1: len 6; hex 000000000902; asc ;; RECORD LOCKS space id 3 page no 9 n bits 320 index GEN_CLUST_INDEX of table `kk`.`k1` trx id 1901 lock_mode X locks rec but not gap Record lock, heap no 248 PHYSICAL RECORD: n_fields 6; compact format; info bits 0 0: len 6; hex 000000000902; asc ;; 1: len 6; hex 000000000663; asc c;; 2: len 7; hex 82000000940110; asc ;; 3: len 4; hex 80000004; asc ;; 4: SQL NULL; 5: SQL NULL; RECORD LOCKS space id 3 page no 10 n bits 1056 index id of table `kk`.`k1` trx id 1901 lock_mode X locks gap before rec Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000016; asc ;; 1: len 6; hex 000000000200; asc ;; .. - space id 3 表的表空间ID - page no 10 锁所在datapage的ID - heap no 4，slot no，记录在page物理上的第几个位置。 -
也可以查询P_S表
mysql> select * from performance_schema.data_locks; +--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+-----------+ | ENGINE | ENGINE_LOCK_ID | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | PARTITION_NAME | SUBPARTITION_NAME | INDEX_NAME | OBJECT_INSTANCE_BEGIN | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+-----------+ | INNODB | 139620969519720:1072:139620864029240 | 2211 | 85 | 79 | kk | k1 | NULL | NULL | NULL | 139620864029240 | TABLE | IX | GRANTED | NULL | | INNODB | 139620969519720:15:4:5:139620864026200 | 2211 | 85 | 79 | kk | k1 | NULL | NULL | PRIMARY | 139620864026200 | RECORD | X,REC_NOT_GAP | GRANTED | 4 | +--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+-----------+ 2 rows in set (0.00 sec) -
再看下IS锁的情况
mysql> begin ; select * from k1 where id=4 for share;
Query OK, 0 rows affected (0.00 sec)
mysql> show engine innodb status G
看不到IS锁信息。
1 row in set (0.00 sec)
mysql> select * from performance_schema.data_locks;
+--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+-----------+
| ENGINE | ENGINE_LOCK_ID | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | PARTITION_NAME | SUBPARTITION_NAME | INDEX_NAME | OBJECT_INSTANCE_BEGIN | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+-----------+
| INNODB | 139620969519720:1072:139620864029240 | 421095946230376 | 85 | 83 | kk | k1 | NULL | NULL | NULL | 139620864029240 | TABLE | IS | GRANTED | NULL |
| INNODB | 139620969519720:15:4:5:139620864026200 | 421095946230376 | 85 | 83 | kk | k1 | NULL | NULL | PRIMARY | 139620864026200 | RECORD | S,REC_NOT_GAP | GRANTED | 4 |
+--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+-----------+
2 rows in set (0.00 sec)
验证一下IS和IX的兼容
t1:
mysql> begin ; select * from k1 where id=4 for share;
Query OK, 0 rows affected (0.00 sec)
+------+------+------+
| id | dtl | name |
+------+------+------+
| 4 | NULL | NULL |
+------+------+------+
1 row in set (0.00 sec)
t2:
mysql> begin ; select * from k1 where id=11 for update;
Query OK, 0 rows affected (0.00 sec)
Empty set (0.00 sec)
--注意:加锁加不在相同行，否则hang。
t3:
mysql> select * from performance_schema.data_locks;
+--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+------------------------+
| ENGINE | ENGINE_LOCK_ID | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | PARTITION_NAME | SUBPARTITION_NAME | INDEX_NAME | OBJECT_INSTANCE_BEGIN | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+------------------------+
| INNODB | 139620969519720:1072:139620864029240 | 2212 | 85 | 89 | kk | k1 | NULL | NULL | NULL | 139620864029240 | TABLE | IX | GRANTED | NULL |
| INNODB | 139620969519720:15:4:1:139620864026200 | 2212 | 85 | 89 | kk | k1 | NULL | NULL | PRIMARY | 139620864026200 | RECORD | X | GRANTED | supremum pseudo-record |
| INNODB | 139620969521464:1072:139620864041176 | 421095946232120 | 84 | 110 | kk | k1 | NULL | NULL | NULL | 139620864041176 | TABLE | IS | GRANTED | NULL |
| INNODB | 139620969521464:15:4:5:139620864038296 | 421095946232120 | 84 | 110 | kk | k1 | NULL | NULL | PRIMARY | 139620864038296 | RECORD | S,REC_NOT_GAP | GRANTED | 4 |
+--------+----------------------------------------+-----------------------+-----------+----------+---------------+-------------+----------------+-------------------+------------+-----------------------+-----------+---------------+-------------+------------------------+
4 rows in set (0.00 sec)
InnoDB行锁实现机制
- 基于索引实现，逐行检查，逐行加锁
- 没有索引的列上需要加锁时，会先对所有记录加锁，再根据实际情况决定是否释放锁。
- 辅助索引上加锁时，同时要回溯到主键索引上再加一次锁。
- 加锁的基本单位默认时lock_ordinary，当索引就具有唯一性的时候退化为lock_rec_not_gap
- 等值条件逐行加锁时，会向右遍历到第一个不满足条件的记录，然后lock_ordinary退化为lock_gap
- 如果发生唯一性检测（insertupdate动作），那么会发生lock_ordinary ， 再退化成lock_rec_not_gap
- 唯一索引的范围条件加锁时，也会对第一个不满足条件的记录加锁
对普通索引上锁
普通索引next-key lock + 主键 not gap + 普通索引的下一个记录的gap lock（见示意图）。
mysql> select * from k2;
+----+------+------+
| id | dtl | un |
+----+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | 5 | 5 | (回溯到pk上锁)
------------------------------------------ <- gap (同时锁住普通索引下一个记录前的gap)
| 4 | 7 | 7 |*
| 5 | 11 | 11 |
+----+------+------+
5 rows in set (0.00 sec)
- 辅助索引上锁的验证实验
mysql> show create table k2G
*************************** 1. row ***************************
Table: k2
Create Table: CREATE TABLE `k2` (
`id` int NOT NULL AUTO_INCREMENT,
`dtl` int DEFAULT NULL,
`un` int DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `dtl` (`dtl`),
KEY `un` (`un`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)
mysql> select * from k2;
+----+------+------+
| id | dtl | un |
+----+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | 5 | 5 |
| 4 | 7 | 7 |*
| 5 | 11 | 11 |
+----+------+------+
5 rows in set (0.00 sec)
mysql> begin ; select * from k2 where un=5 for update;
Query OK, 0 rows affected (0.00 sec)
+----+------+------+
| id | dtl | un |
+----+------+------+
| 3 | 5 | 5 |
+----+------+------+
1 row in set (0.00 sec)
mysql> select * from performance_schema.data_locksG
*************************** 1. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139620969521464:1061:139620864041176
ENGINE_TRANSACTION_ID: 1944
THREAD_ID: 70
EVENT_ID: 65
OBJECT_SCHEMA: kk
OBJECT_NAME: k2
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: NULL
OBJECT_INSTANCE_BEGIN: 139620864041176
LOCK_TYPE: TABLE*
LOCK_MODE: IX*
LOCK_STATUS: GRANTED
LOCK_DATA: NULL
*************************** 2. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139620969521464:4:6:9:139620864038296
ENGINE_TRANSACTION_ID: 1944
THREAD_ID: 70
EVENT_ID: 65
OBJECT_SCHEMA: kk
OBJECT_NAME: k2
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: un*
OBJECT_INSTANCE_BEGIN: 139620864038296
LOCK_TYPE: RECORD （nextkey-lock）*
LOCK_MODE: X
LOCK_STATUS: GRANTED
LOCK_DATA: 5, 3 （un key value, primary key value, index Condion特性）*
*************************** 3. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139620969521464:4:4:11:139620864038640
ENGINE_TRANSACTION_ID: 1944
THREAD_ID: 70
EVENT_ID: 65
OBJECT_SCHEMA: kk
OBJECT_NAME: k2
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: PRIMARY*
OBJECT_INSTANCE_BEGIN: 139620864038640
LOCK_TYPE: RECORD
LOCK_MODE: X,REC_NOT_GAP*
LOCK_STATUS: GRANTED
LOCK_DATA: 3*
*************************** 4. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139620969521464:4:6:10:139620864038984
ENGINE_TRANSACTION_ID: 1944
THREAD_ID: 70
EVENT_ID: 65
OBJECT_SCHEMA: kk
OBJECT_NAME: k2
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: un*
OBJECT_INSTANCE_BEGIN: 139620864038984
LOCK_TYPE: RECORD
LOCK_MODE: X,GAP* 确认下一个记录不符合条件，回退nextkey-lock 为 lock-gap.
LOCK_STATUS: GRANTED
LOCK_DATA: 7, 4 （un key value, primary key value, index Condion特性）*
4 rows in set (0.00 sec)
InnoDB隐式、显式锁
-
显式锁(explicit-lock)
- select .. from .. where .. for update / for share
-
隐式锁(implicit-lock)
- update set .. where ..
- 任何辅助索引上锁，或非索引列上锁，都要回溯到主键上再加锁。
- 和其他session有冲突时，隐式锁转换为显式锁。（没实验验证出来。）
如果发生唯一性检测（insertupdate动作），那么会发生lock_ordinary ， 再退化成lock_rec_not_gap