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| MariaDB [db1]> create table t1 (a int primary key);
Query OK, 0 rows affected (0.22 sec)
MariaDB [db1]> insert into t1 values (1),(3),(5);
Query OK, 3 rows affected (0.02 sec)
Records: 3 Duplicates: 0 Warnings: 0
#事务T1
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> set session transaction isolation level read co
Query OK, 0 rows affected (0.01 sec)
MariaDB [db1]> select * from db1.t1 where a>3 for update;
+---+
| a |
+---+
| 5 |
+---+
1 row in set (0.01 sec)
#事务T2
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> insert into db1.t1 values (4);
Query OK, 1 row affected (0.00 sec)
MariaDB [db1]> commit;
Query OK, 0 rows affected (0.03 sec)
#事务T1
MariaDB [db1]> select * from db1.t1 where a>3 for update;
+---+
| a |
+---+
| 4 |
| 5 |
+---+
2 rows in set (0.00 sec)
|
将会话中的隔离界别改为RR,并删除a=4记录。
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| MariaDB [db1]> set session transaction isolation level repeatable read;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> delete from db1.t1 where a=4;
Query OK, 1 row affected (0.00 sec)
|
T1 RR
T2 RR
begin;begin;
select * from db1.t1 where a>3 for update;
查询结果为5
insert into db1.t1 values (4);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
commit;
select * from db1.t1 where a>3;
查询结果为5
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| #事务T1
MariaDB [(none)]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> select * from db1.t1 where a>3 for update;
+---+
| a |
+---+
| 5 |
+---+
1 row in set (0.02 sec)
#事务T2
MariaDB [(none)]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> insert into db1.t1 values (4);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
MariaDB [(none)]> commit;
Query OK, 0 rows affected (0.00 sec)
#事务T1
MariaDB [(none)]> select * from db1.t1 where a>3 for update;
+---+
| a |
+---+
| 5 |
+---+
1 row in set (0.02 sec)
|
认识锁的算法nnoDB存储引擎的锁的算法有三种:
- Record lock:单个行记录上的锁
- Gap lock:间隙锁,锁定一个范围,不包括记录本身
- Next-key lock:record+gap 锁定一个范围,包含记录本身
Lock的精度(type)分为 行锁、表锁、意向锁
Lock的模式(mode)分为:
- 锁的类型 ——【读锁和写锁】或者【共享锁和排他锁】即 【X or S】
- 锁的范围 ——【record lock、gap lock、Next-key lock】
知识点- innodb对于行的查询使用next-key lock
- Next-locking keying为了解决Phantom Problem幻读问题
- 当查询的索引含有唯一属性时,将next-key lock降级为record key
- Gap锁设计的目的是为了阻止多个事务将记录插入到同一范围内,而这会导致幻读问题的产生
- 有两种方式显式关闭gap锁:(除了外键约束和唯一性检查外,其余情况仅使用record lock)A. 将事务隔离级别设置为RCB. 将参数innodb_locks_unsafe_for_binlog设置为1
实践1: 验证next-key lock降级为record key创建db1.t1表,有列a和b,分别为char(10)和int型,并且b为key,注意b列为索引列,但并不是主键,因此不是唯一的。
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| MariaDB [db1]> create table db1.t1 (a char(10),b int,key (b));
Query OK, 0 rows affected (0.03 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',1),('superman',3),('leo',5);
Query OK, 3 rows affected (0.15 sec)
Records: 3 Duplicates: 0 Warnings: 0
MariaDB [db1]> select * from db1.t1;
+----------+------+
| a | b |
+----------+------+
| batman | 1 |
| superman | 3 |
| leo | 5 |
+----------+------+
3 rows in set (0.02 sec)
|
接下来开启两个事务T1和T2,T1中查看b=3的行,显式加排他锁;T1未提交事务时,T2事务开启并尝试插入新行a='batman',b=2和a='batman',b=4;
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| #事务T1
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> select * from db1.t1 where b=3 for update;
+----------+------+
| a | b |
+----------+------+
| superman | 3 |
+----------+------+
1 row in set (0.12 sec)
#事务T2
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',2);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
MariaDB [db1]> insert into db1.t1 values ('batman',4);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
|
发现T2事务中不能插入新行a='batman',b=2和a='batman',b=4;可以查看当前innodb锁的信息
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| MariaDB [db1]> select * from information_schema.innodb_locks\G;
*************************** 1. row ***************************
lock_id: 111B:0:334:3
lock_trx_id: 111B
lock_mode: X,GAP
lock_type: RECORD
lock_table: `db1`.`t1`
lock_index: `b`
lock_space: 0
lock_page: 334
lock_rec: 3
lock_data: 3, 0x00000000020E
*************************** 2. row ***************************
lock_id: 111A:0:334:3
lock_trx_id: 111A
lock_mode: X
lock_type: RECORD
lock_table: `db1`.`t1`
lock_index: `b`
lock_space: 0
lock_page: 334
lock_rec: 3
lock_data: 3, 0x00000000020E
2 rows in set (0.01 sec)
ERROR: No query specified
MariaDB [db1]> select * from information_schema.innodb_lock_waits\G;
*************************** 1. row ***************************
requesting_trx_id: 111B
requested_lock_id: 111B:0:334:3
blocking_trx_id: 111A
blocking_lock_id: 111A:0:334:3
1 row in set (0.09 sec)
MariaDB [db1]> select * from information_schema.innodb_lock_waits\G;
*************************** 1. row ***************************
requesting_trx_id: 111B
requested_lock_id: 111B:0:334:4
blocking_trx_id: 111A
blocking_lock_id: 111A:0:334:4
1 row in set (0.00 sec)
ERROR: No query specified
MariaDB [db1]> select * from information_schema.innodb_locks\G;
*************************** 1. row ***************************
lock_id: 111B:0:334:4
lock_trx_id: 111B
lock_mode: X,GAP
lock_type: RECORD
lock_table: `db1`.`t1`
lock_index: `b`
lock_space: 0
lock_page: 334
lock_rec: 4
lock_data: 5, 0x00000000020F
*************************** 2. row ***************************
lock_id: 111A:0:334:4
lock_trx_id: 111A
lock_mode: X,GAP
lock_type: RECORD
lock_table: `db1`.`t1`
lock_index: `b`
lock_space: 0
lock_page: 334
lock_rec: 4
lock_data: 5, 0x00000000020F
2 rows in set (0.11 sec)
ERROR: No query specified
|
我们看到T2事务的两次插入动作都在请求排他锁,但是此时T1事务已经在加了next-key lock(record + gap),表现范围为b的(1,5),包括记录3,所以T2事务在T1事务解锁之间,不能插入到b的(1,5)范围内
* lock_mode: X,GAP lock_mode 可以理解为 读锁还是写锁?;是在什么范围上锁?;此处加的写锁即排他锁;范围是(1,5)
* lock_type: RECORD 表示锁的精度,根据存储引擎不同,innodb是行锁,MYISAM是表锁
删除db1.t1表,重新创建db1.t1表,有列a和b,分别为char(10)和int型,并且b为primay key,因此b列是唯一的。
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| MariaDB [db1]> drop tables t1;
Query OK, 0 rows affected (0.12 sec)
MariaDB [db1]> create table db1.t1 (a char(10),b int ,primary key (b));
Query OK, 0 rows affected (0.02 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',1),('superman',3),('leo',5);
Query OK, 3 rows affected (0.12 sec)
Records: 3 Duplicates: 0 Warnings: 0
MariaDB [db1]> select * from db1.t1;
+----------+---+
| a | b |
+----------+---+
| batman | 1 |
| superman | 3 |
| leo | 5 |
+----------+---+
3 rows in set (0.08 sec)
|
接下来开启两个事务T1和T2,T1中查看b=3的行,显式加排他锁;T1未提交事务时,T2事务开启并尝试插入新行a='batman',b=2和a='batman',b=4;
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| #事务T1
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> select * from db1.t1 where b=3 for update;
+----------+---+
| a | b |
+----------+---+
| superman | 3 |
+----------+---+
1 row in set (0.14 sec)
#事务T2
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',2);
Query OK, 1 row affected (0.00 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',4);
Query OK, 1 row affected (0.00 sec)
|
继续在T2事务中尝试查看b=3的行,显式加共享锁。
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| #事务T2
MariaDB [db1]> select * from db1.t1 where b=3 lock in share mode;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
|
发现T2事务中可以插入新行a='batman',b=2和a='batman',b=4;但是不能查看b=3的行,接下来我们查看当前innodb锁的信息
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| MariaDB [db1]> select * from information_schema.innodb_locks\G;
*************************** 1. row ***************************
lock_id: 1122:0:337:3
lock_trx_id: 1122
lock_mode: S
lock_type: RECORD
lock_table: `db1`.`t1`
lock_index: `PRIMARY`
lock_space: 0
lock_page: 337
lock_rec: 3
lock_data: 3
*************************** 2. row ***************************
lock_id: 1121:0:337:3
lock_trx_id: 1121
lock_mode: X
lock_type: RECORD
lock_table: `db1`.`t1`
lock_index: `PRIMARY`
lock_space: 0
lock_page: 337
lock_rec: 3
lock_data: 3
2 rows in set (0.02 sec)
ERROR: No query specified
MariaDB [db1]> select * from information_schema.innodb_lock_waits\G;
*************************** 1. row ***************************
requesting_trx_id: 1122
requested_lock_id: 1122:0:337:3
blocking_trx_id: 1121
blocking_lock_id: 1121:0:337:3
1 row in set (0.00 sec)
ERROR: No query specified
|
从以上信息可以看到,T1事务当前只在b=3所在的行上加了写锁,排他锁,并没有同时使用gap锁来组成next-key lock。
到此,已经证明了,当查询的索引含有唯一属性时,将next-key lock降级为record key
我们第二次创建的t1表的列b是主键,而主键必须是唯一的。
实践2: 关闭GAP锁_RC有两种方式显式关闭gap锁:(除了外键约束和唯一性检查外,其余情况仅使用record lock)
A. 将事务隔离级别设置为RCB. 将参数innodb_locks_unsafe_for_binlog设置为1
T1 RR
T2 RR
begin;
begin;
select * from db1.t1 where b=3 for update;
insert into db1.t1 values ('batman',2)
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
set session transaction isolation level READ COMMITTED;
commit;
commit;
注意,将T1事务设置为RC后,需要将二进制日志的格式改为row格式,否则执行显式加锁时会报错
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| MariaDB [db1]> insert into t1 values ('batman',2);
ERROR 1665 (HY000): Cannot execute statement: impossible to write to binary log since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine limited to row-based logging. InnoDB is limited to row-logging when transaction isolation level is READ COMMITTED or READ UNCOMMITTED.
|
T1 RC
T2 RR
begin;
begin;
set session transaction isolation level READ COMMITTED;
select * from db1.t1 where b=3 for update;
insert into db1.t1 values ('batman',2)
insert into db1.t1 values ('batman',4)
commit;
commit;
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| #T1事务
MariaDB [db1]> set session transaction isolation level READ COMMITTED;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.09 sec)
MariaDB [db1]> select * from t1 where b=3 for update;
+----------+------+
| a | b |
+----------+------+
| superman | 3 |
+----------+------+
1 row in set (0.00 sec)
#T2事务
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.16 sec)
MariaDB [db1]> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',2);
Query OK, 1 row affected (0.00 sec)
MariaDB [db1]> commit;
Query OK, 0 rows affected (0.01 sec)
MariaDB [db1]> set session transaction isolation level REPEATABLE READ;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
MariaDB [db1]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',4);
Query OK, 1 row affected (0.00 sec)
MariaDB [db1]> commit;
Query OK, 0 rows affected (0.00 sec)
#T1事务
MariaDB [db1]> commit;
Query OK, 0 rows affected (0.00 sec)
|
我在做测试的时候,T1事务隔离界别为RC,T2事务的隔离界别分别用RC和RR做了测试,都是可以的
实践3: 关闭GAP锁_innodb_locks_unsafe_for_binlog
查看当前innodb_locks_unsafe_for_binlog参数的值
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| MariaDB [(none)]> select @@innodb_locks_unsafe_for_binlog;
+----------------------------------+
| @@innodb_locks_unsafe_for_binlog |
+----------------------------------+
| 0 |
+----------------------------------+
1 row in set (0.00 sec)
|
修改参数,并重新启动服务
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| [iyunv@localhost ~]# vim /etc/my.cnf
innodb_locks_unsafe_for_binlog=1
[iyunv@localhost ~]# systemctl restart mariadb
[iyunv@localhost ~]# mysql -e "select @@innodb_locks_unsafe_for_binlog"
+----------------------------------+
| @@innodb_locks_unsafe_for_binlog |
+----------------------------------+
| 1 |
+----------------------------------+
|
还是去创建db1.t1表,如果已有就先drop;有列a和b,分别为char(10)和int型,并且b为key,注意b列为索引列,但并不是主键,因此不是唯一的。
T1 RR
T2 RR
begin;
begin;
select * from db1.t1 where b=3 for update;
insert into db1.t1 values ('batman',2)
insert into db1.t1 values ('batman',4)
commit;
commit;
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| MariaDB [db1]> create table db1.t1 (a char(10),b int,key (b));
Query OK, 0 rows affected (0.03 sec)
MariaDB [db1]> insert into db1.t1 values ('batman',1),('superman',3),('leo',5);
Query OK, 3 rows affected (0.15 sec)
Records: 3 Duplicates: 0 Warnings: 0
MariaDB [db1]> select * from db1.t1;
+----------+------+
| a | b |
+----------+------+
| batman | 1 |
| superman | 3 |
| leo | 5 |
+----------+------+
3 rows in set (0.02 sec)
|
接下来开启两个事务T1和T2,T1中查看b=3的行,显式加排他锁;T1未提交事务时,T2事务开启并尝试插入新行a='batman',b=2和a='batman',b=4;
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| #T1事务
MariaDB [(none)]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> select * from db1.t1 where b=3 for update;
+----------+------+
| a | b |
+----------+------+
| superman | 3 |
+----------+------+
1 row in set (0.01 sec)
#T2事务
MariaDB [(none)]> begin;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> insert into db1.t1 values ('batman',4);
Query OK, 1 row affected (0.01 sec)
MariaDB [(none)]> insert into db1.t1 values ('batman',2);
Query OK, 1 row affected (0.00 sec)
MariaDB [(none)]> commit;
Query OK, 0 rows affected (0.00 sec)
#T1事务
MariaDB [(none)]> commit;
Query OK, 0 rows affected (0.00 sec)
|