MySQL的在RC和RR模式下的锁
InnoDB的锁机制：
数据库使用所是为了支持更好的并发，提供数据的完整性和一致性。InnoDB是一个支持锁的存储引擎，锁的类型有：共享锁（S）、排它锁（X）、意向共享锁（IS）、意向排它锁（IX）。为了支持更好的并发，InnoDB提供了非锁定读：不需要等待访问行上的锁释放，读取行的一个快照。该方法是通过InnoDB的一个特写：MVCC实现的。
InnoDB的锁分类：
Record Lock：行锁：单个行记录上的行锁
Gap Lock：间隙锁，锁定一个范围，但不包括记录本身
Next-Key Lock：Gap+Record Lock，锁定一个范围，并且锁定记录本身
无索引+RC/RR
当对无索引的字段进行更新时（RR级别），通过锁主键的方式，来锁住所有记录，RC级别不会锁所有记录。
构建表及初始化数据：
mysql -uroot -p USE test; DROP TABLE IF EXISTS t_none; CREATE TABLE `t_none` ( `id` int(11) NOT NULL, `mem_id` int(11) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB; INSERT INTO t_none VALUES(1,1),(3,3),(5,5),(9,9),(11,11);
REPEATABLE-READ（RR）默认级别 | |
Session A | Session B |
root@localhost[zjkj]:10:53:18>prompt A>> PROMPT set to 'A>>' A>>select @@session.tx_isolation; | root@localhost[(none)]:11:02:58>prompt B>> PROMPT set to 'B>>' B>>select @@session.tx_isolation; |
A>>begin; Query OK, 0 rows affected (0.00 sec) | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_none; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) | B>>select * from t_none; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) |
A>> select * from t_none where mem_id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 3 | 3 | +----+--------+ 1 row in set (0.01 sec) | |
B>>insert into t_none values(2,2); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction B>>delete from t_none where id=9; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction | |
show engin inondb status部分输出： ------------ TRANSACTIONS ------------ Trx id counter 10661 Purge done for trx's n:o < 10659 undo n:o < 0 state: running but idle History list length 351 Total number of lock structs in row lock hash table 2 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 10588, not started MySQL thread id 4, OS thread handle 0x7f6f5085c700, query id 339 localhost root init show engine innodb status ---TRANSACTION 10660, ACTIVE 17 sec inserting mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 376, 1 row lock(s) MySQL thread id 11, OS thread handle 0x7f6f508de700, query id 338 localhost root update insert into t_none values(2,2) ------- TRX HAS BEEN WAITING 17 SEC FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 68 page no 3 n bits 72 index `PRIMARY` of table `test`.`t_none` trx id 10660 lock_mode X locks gap before rec insert intention waiting | |
结论：通过上面很容易的看到，没有通过索引for update时，当进行增删改都会锁住，MySQL内部会通过基于锁默认主键方式，对所有记录加X锁。 下面是RC级别的实验 | |
Read Committed级别（RC） | |
Session A | Session B |
A>>set @@session.tx_isolation="read-committed"; Query OK, 0 rows affected (0.00 sec) | B>>set @@session.tx_isolation="read-committed"; Query OK, 0 rows affected (0.00 sec) |
A>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-COMMITTED | +------------------------+ 1 row in set (0.00 sec) | B>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-COMMITTED | +------------------------+ 1 row in set (0.01 sec) |
A>>begin; Query OK, 0 rows affected (0.00 sec) | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_none where mem_id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 3 | 3 | +----+--------+ 1 row in set (0.01 sec) | |
B>>insert into t_none values(2,2); Query OK, 1 row affected (0.01 sec) | |
B>>select * from t_none; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 6 rows in set (0.00 sec | |
A>>rollback; Query OK, 0 rows affected (0.00 sec) | B>>rollback; Query OK, 0 rows affected (0.00 sec) |
结论：在RC级别下，事务B是可以进行增删改（除被锁定的记录本身） | |
非唯一索引+RR/RC
在RR级别下，InnoDB对于非唯一索引会加Gap Lock（也即锁定一个区间），而在RC级别下无。
构造初始化表及数据：
mysql -uroot -p USE test; DROP TABLE IF EXISTS t_idx; CREATE TABLE `t_idx` ( `id` int(11) NOT NULL, `mem_id` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_mem_id` (`mem_id`) ) ENGINE=InnoDB; INSERT INTO t_idx VALUES(1,1),(3,3),(5,5),(9,9),(11,11);
| REPEATABLE-READ（RR）默认级别（RR模式） | |
Session A | Session B |
root@localhost[(none)]:06:01:59>use test; root@localhost[zjkj]:10:53:18>prompt A>> PROMPT set to 'A>>' | root@localhost[(none)]:06:01:59>use test; root@localhost[(none)]:11:02:58>prompt B>> PROMPT set to 'B>>' |
A>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | REPEATABLE-READ | +------------------------+ 1 row in set (0.00 sec) | B>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | REPEATABLE-READ | +------------------------+ 1 row in set (0.02 sec) |
A>>begin; Query OK, 0 rows affected (0.00 sec) | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_idx; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.04 sec) | B>>select * from t_idx; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) |
A>>select * from t_idx where mem_id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 3 | 3 | +----+--------+ 1 row in set (0.05 sec) | |
B>>insert into t_idx values(2,2); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction #问题？这里为什么会出现阻塞呢？ B>>insert into t_idx values(4,4); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction #问题？这里为什么会出现阻塞呢？ B>>insert into t_idx values(3,3); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction B>>insert into t_idx values(5,5); ERROR 1062 (23000): Duplicate entry '5' for key 'PRIMARY' B>>insert into t_idx values(1,1); ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY' #######下面插入全部可以###### B>>insert into t_idx values(6,6); Query OK, 1 row affected (0.00 sec) B>>insert into t_idx values(7,7); B>>insert into t_idx values(8,8); Query OK, 1 row affected (0.01 sec) B>>insert into t_idx values(12,12); Query OK, 1 row affected (0.00 sec) | |
B>>select * from t_idx; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 6 | 6 | | 7 | 7 | | 8 | 8 | | 9 | 9 | | 11 | 11 | | 12 | 12 | +----+--------+ 9 rows in set (0.00 sec) | |
| show engine inondb status部分输出： ------------ TRANSACTIONS ------------ Trx id counter 11044 Purge done for trx's n:o < 11041 undo n:o < 0 state: running but idle History list length 372 Total number of lock structs in row lock hash table 5 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0, not started MySQL thread id 3, OS thread handle 0x7fd0430df700, query id 47 localhost root init show engine innodb status ---TRANSACTION 11039, ACTIVE 228 sec inserting mysql tables in use 1, locked 1 LOCK WAIT 3 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 4 MySQL thread id 1, OS thread handle 0x7fd064099700, query id 45 localhost root update insert into t_idx values(4,4) Trx read view will not see trx with id >= 11040, sees < 11038 ------- TRX HAS BEEN WAITING 22 SEC FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 70 page no 4 n bits 80 index `idx_mem_id` of table `test`.`t_idx` trx id 11039 lock_mode X locks gap before rec insert intention waitin | |
| 结论：通过上面可以看到，通过非唯一索引字段进行更新时，在进行增删改时，有的记录会出现阻塞，为什么会出现阻塞呢？其实就是用到了MySQL的间隙锁。那MySQL这里为什么要用间隙锁呢？目的主要是防止幻读。 那为什么有的记录可以插入有的不可以，因为InnoDB对于行的查询时采用了Next-Key Lock的算法，锁定的是一个范围（GAP）如下：（∞,1]，(1,3]，(3,5]，(5,9]，(9,11]，(11, ∞)。InnoDB对辅助索引下一个键值也要加上Gap Lock，例如上面进行插入2、4、1、3、5时，就可以看出，其实锁住的区间是(1,5)。 | |
| Read Committed级别（RC） | |
Session A | Session B |
A>>rollback; Query OK, 0 rows affected (0.00 sec) | B>>rollback; Query OK, 0 rows affected (0.00 sec) |
A>>set @@session.tx_isolation="read-committed"; Query OK, 0 rows affected (0.00 sec) | B>>set @@session.tx_isolation="read-committed"; Query OK, 0 rows affected (0.00 sec) |
A>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-COMMITTED | +------------------------+ 1 row in set (0.00 sec) | B>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-COMMITTED | +------------------------+ 1 row in set (0.01 sec) |
A>>begin; Query OK, 0 rows affected (0.00 sec) | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_idx where mem_id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 1 | 3 | | 3 | 3 | +----+--------+ 2 rows in set (0.00 sec) | |
B>>insert into t_idx values(1,1); ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY' B>>insert into t_idx values(2,2); Query OK, 1 row affected (0.00 sec) B>>insert into t_idx values(3,3); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction B>>insert into t_idx values(4,4); Query OK, 1 row affected (0.01 sec) | |
| 结论：在RC级别下，事务B是可以进行增删改（除被锁定的记录本身），没有出现间隙锁的现象。 | |
唯一索引+RR/RC
构造初始化表及数据：
mysql -uroot –p use test; DROP TABLE IF EXISTS t_pk; CREATE TABLE `t_pk` ( `id` int(11) NOT NULL AUTO_INCREMENT, `mem_id` int(11) NOT NULL , PRIMARY KEY (`id`), UNIQUE `uq_mem_id` (`mem_id`) ) ENGINE=InnoDB; INSERT INTO t_pk VALUES(1,1),(3,3),(5,5),(9,9),(11,11);
| REPEATABLE READ（RR级别） | |
root@localhost[(none)]:10:04:34>use test; root@localhost[test]:10:04:41>prompt A>> PROMPT set to 'A>>' | root@localhost[(none)]:10:04:37>use test; root@localhost[test]:10:04:52>prompt B>> PROMPT set to 'B>>' |
A>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | REPEATABLE-READ | +------------------------+ 1 row in set (0.01 sec) | B>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | REPEATABLE-READ | +------------------------+ 1 row in set (0.00 sec) |
A>>begin; Query OK, 0 rows affected (0.00 sec) | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_pk; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) | B>>select * from t_pk; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) |
A>>select * from t_pk where mem_id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 3 | 3 | +----+--------+ 1 row in set (0.00 sec) | |
B>>insert into t_pk values(2,2); Query OK, 1 row affected (0.00 sec) B>>insert into t_pk values(4,4); Query OK, 1 row affected (0.00 sec) B>>insert into t_pk values(3,3); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction B>>insert into t_pk values(5,5); ERROR 1062 (23000): Duplicate entry '5' for key 'PRIMARY' B>>insert into t_pk values(7,7); Query OK, 1 row affected (0.00 sec) | |
| 结论：从这里可以看到，对于基于唯一索引的更新，MySQL只是锁定了记录本身。 同理，我们可以推导出主键也是一样的。实验的话我就略了，其实就是将上面的mem_id改成id即可。 | |
| 基于主键的Record Lock，还是RR级别 | |
A>>rollback; Query OK, 0 rows affected (0.00 sec) | B>>rollback; Query OK, 0 rows affected (0.00 sec) |
A>>begin; Query OK, 0 rows affected (0.00 sec | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_pk where id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 3 | 3 | +----+--------+ 1 row in set (0.00 sec) | |
B>>insert into t_pk values(2,2); Query OK, 1 row affected (0.00 sec) B>>insert into t_pk values(4,4); Query OK, 1 row affected (0.00 sec) | |
| 结论：说明上面的推导正确。 | |
| Read-Committed级别（RC） | |
A>>rollback; Query OK, 0 rows affected (0.00 sec) | B>>rollback; Query OK, 0 rows affected (0.00 sec) |
A>>set @@session.tx_isolation="read-committed"; Query OK, 0 rows affected (0.01 sec) | B>>set @@session.tx_isolation="read-committed"; Query OK, 0 rows affected (0.00 sec) |
A>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-COMMITTED | +------------------------+ 1 row in set (0.00 sec) | B>>select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-COMMITTED | +------------------------+ 1 row in set (0.00 sec) |
A>>begin; Query OK, 0 rows affected (0.00 sec) | B>>begin; Query OK, 0 rows affected (0.00 sec) |
A>>select * from t_pk; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) | B>>select * from t_pk; +----+--------+ | id | mem_id | +----+--------+ | 1 | 1 | | 3 | 3 | | 5 | 5 | | 9 | 9 | | 11 | 11 | +----+--------+ 5 rows in set (0.00 sec) |
A>>select * from t_pk where mem_id=3 for update; +----+--------+ | id | mem_id | +----+--------+ | 3 | 3 | +----+--------+ 1 row in set (0.00 sec) | |
B>>insert into t_pk values(2,2); Query OK, 1 row affected (0.00 sec) B>>insert into t_pk values(4,4),(6,6),(10,10); Query OK, 3 rows affected (0.00 sec) Records: 3 Duplicates: 0 Warnings: 0 | |
| 结论:说明RC级别下，没有间隙锁存在。 | |
主键+RR/RC
这跟唯一索引+RR/RC是一样的，请参看上面的唯一索引+RR/RC。
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