Next-Key Locks
一个next-key lock 是 一个record lock 在index record 和 一个区间锁 在一个区间在index record之前
InnoDB 执行 row-level locking 以这样一种方式当它搜索或者扫描 一个表的索引,
它设置共享或者排它锁在index records.
因此, row-level locks 实际上是 index-record locks.
一个next-key lock 在一个index record 也影响区间在那个index record 之前。
也就是说,一个next-key lock 是一个Index-record 加上一个区间锁在index record 之前的区间。
如果一个会话有一个共享或者排它锁在记录R上在一个索引上,
另外的会话不能插入一个新的index record 在这个区间
假设一个Index 包含值10,11,13,20.
可能的next-key locks 对于这个index包含了下面的时间间隔,
一个圆括号表示排除间隔端点
一个方括号表示包含间隔端点
(negative infinity, 10]
(10, 11]
(11, 13]
(13, 20]
(20, positive infinity)
在最后的区间, next-key lock locks 的区间在最大值的上界在index和上界的伪记录
有一个值高于任何值在index里。
上界限不是一个真正的index record.所以,实际上,
next-key lock locks只有区间在最大索引值后面的区间
默认情况下, InnoDB 工作在REPEATABLE READ 事务隔离级别下
innodb_locks_unsafe_for_binlog 系统变量被禁用,
在那种情况下,可以使用next-key locks来搜索和索引扫描,来防止幻读行
Session 1:
mysql> explain select * from t1 where id BETWEEN 5 and 7 for update;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------------+
| 1 | SIMPLE | t1 | range | t1_idx1 | t1_idx1 | 5 | NULL | 3 | Using index condition |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-----------------------+
1 row in set (0.00 sec)
mysql> select * from t1 where id BETWEEN 5 and 7 for update;
+-----+------+------+
| sn | id | info |
+-----+------+------+
| 239 | 5 | a5 |
| 240 | 6 | a6 |
| 241 | 7 | a7 |
+-----+------+------+
3 rows in set (0.00 sec)
Session 2:
mysql> update t1 set id=300 where id=3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
mysql> update t1 set id=500 where id=5;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t1 set id=600 where id=6;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t1 set id=600 where id=4;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
mysql> update t1 set id=600 where id=7;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t1 set id=800 where id=8;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t1 set id=800 where id=9;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
会锁住 5,6,7,8 4条记录
继续测试:
Session 1:
mysql> show index from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | sn | A | 11 | NULL | NULL | | BTREE | | |
| t1 | 0 | t1_idx1 | 1 | id | A | 11 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql> select * from t1 where id=7 for update;
+-----+------+------+
| sn | id | info |
+-----+------+------+
| 241 | 7 | a7 |
+-----+------+------+
1 row in set (0.00 sec)
Session 2:
mysql> update t1 set id=800 where id=8;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
去掉Id列的索引继续测试:
Session 1:
mysql> show index from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | sn | A | 11 | NULL | NULL | | BTREE | | |
| t1 | 0 | t1_idx1 | 1 | id | A | 11 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql> alter table t1 drop index t1_idx1;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show index from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | sn | A | 11 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
mysql> select * from t1 where id=7 for update;
+-----+------+------+
| sn | id | info |
+-----+------+------+
| 241 | 7 | a7 |
+-----+------+------+
1 row in set (0.00 sec)
Session 2:
Database changed
mysql> update t1 set id=800 where id=8; --HANG
/************************************************
mysql> explain select * from t1 where id=7 for update;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 11 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> show index from t1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t1 | 0 | PRIMARY | 1 | sn | A | 11 | NULL | NULL | | BTREE | | |
| t1 | 1 | t1_idx1 | 1 | id | A | 11 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql> explain select * from t1 where id=7 for update;
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | t1 | ref | t1_idx1 | t1_idx1 | 5 | const | 1 | NULL |
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
下面来举个手册上的例子看什么是next-key lock。假如一个索引的行有10,11,13,20
那么可能的next-key lock的包括：
(无穷小, 10]
(10,11]
(11,13]
(13,20]
(20, 无穷大) (这里无穷大为什么不是闭合？你数学不到家~~)
举例测试:
mysql> desc t100;
+-------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+----------------+
| sn | int(11) | NO | PRI | NULL | auto_increment |
| id | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> show create table t100G;
*************************** 1. row ***************************
Table: t100
Create Table: CREATE TABLE `t100` (
`sn` int(11) NOT NULL AUTO_INCREMENT,
`id` int(11) DEFAULT NULL,
PRIMARY KEY (`sn`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
ERROR:
No query specified
Session 1:
mysql> mysql> show index from t100;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t100 | 0 | PRIMARY | 1 | sn | A | 11 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
mysql> select * from t100;
+----+------+
| sn | id |
+----+------+
| 1 | 7 |
| 2 | 9 |
| 3 | 10 |
| 4 | 12 |
| 5 | 13 |
| 6 | 14 |
| 7 | 15 |
| 8 | 22 |
| 9 | 23 |
| 10 | 24 |
| 11 | 25 |
+----+------+
11 rows in set (0.00 sec)
mysql> update t100 set id=100 where id=1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
id列上没有索引,导致:
Session 2:
mysql> insert into t100(id) values(100);
t100表所有记录锁住
/***************
mysql> show index from t100;
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t100 | 0 | PRIMARY | 1 | sn | A | 11 | NULL | NULL | | BTREE | | |
| t100 | 1 | t1oo_idx1 | 1 | id | A | 11 | NULL | NULL | YES | BTREE | | |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
mysql> select * from t100;
+----+------+
| sn | id |
+----+------+
| 1 | 7 |
| 2 | 9 |
| 3 | 10 |
| 4 | 12 |
| 5 | 13 |
| 6 | 14 |
| 7 | 15 |
| 8 | 22 |
| 9 | 23 |
| 10 | 24 |
| 11 | 25 |
+----+------+
11 rows in set (0.00 sec)
mysql> delete from t100 where id=21;
Query OK, 0 rows affected (0.00 sec)
Session 2:
mysql> insert into t100(id) values (15);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t100(id) values (16);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t100(id) values (17);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> mysql> insert into t100(id) values (18);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t100(id) values (19);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t100(id) values (20);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t100(id) values (21);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
锁15-21 之间
行锁加锁对象永远是索引记录，因为innodb中表即索引
在（三）种，a=21也是不存在，但是在表里面21前后都有记录，因此这里next-key lock的区间也就是(15,21]，因此不在这个区间内的都可以插入。
记录锁---锁单条记录；区间锁---锁一个开区间；next-key 锁---前面两者的结合