本文简单介绍了PostgreSQL Locks(Advisory Locks)的基础知识,原文详见[Postgres Locking Revealed](https://engineering.nordeus.com/postgres-locking-revealed/),有所删减和调整. PostgreSQL把锁分为三类,table-level, row-level and advisory locks.Table和Row级锁可以是显式或隐式锁,advisory locks通常是显式锁.显式锁在显式用户请求时获得,而隐式锁则通过标准SQL命令获得. **Advisory Locks** PG提供了应用自定义的locks,这种locks称为Advisory Locks -- 系统并没有强制使用这些锁,由应用自行正确使用. 有两种方法请求获取advisory lock : Session Level和Transaction Level. session level : 一旦在session level获取了锁,除非session终止或者显式的释放,否则锁会一直持有.不同于标准的锁请求,session level的advisory lock不需要遵守事务语义:在事务期间申请的lock会在rollback后仍会持有,同时就算事务调用失败unlock也是有效的.lock可以多次获取,对于每一次完整的锁请求必须在释放锁前有相应的unlock. transaction level : 与普通锁类似,在事务结束时自动释放,没有显式的unlock操作. 对于同一个Advisory Lock,如果同时请求session leve和transaction level的锁,那么会以期望的方式阻塞. 下面是Advisory Lock的例子: session 1 ``` [local]:5432 pg12@testdb=# -- Transaction 1 [local]:5432 pg12@testdb=# BEGIN; BEGIN Time: 0.882 ms [local]:5432 pg12@testdb=#* [local]:5432 pg12@testdb=#* SELECT pg_advisory_xact_lock(1); pg_advisory_xact_lock ----------------------- (1 row) Time: 2.449 ms [local]:5432 pg12@testdb=#* -- Some work here [local]:5432 pg12@testdb=#* ``` session 2 ``` [local]:5432 pg12@testdb=# -- Transaction 2 [local]:5432 pg12@testdb=# BEGIN; BEGIN Time: 0.468 ms [local]:5432 pg12@testdb=#* [local]:5432 pg12@testdb=#* SELECT pg_advisory_xact_lock(1); ``` session 3 ``` [local]:5432 pg12@testdb=# SELECT * FROM pg_locks where pid <> pg_backend_pid(); locktype | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid | mode | granted | fastpath ------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+------+---------------+---------+---------- virtualxid | | | | | 5/4 | | | | | 5/4 | 1789 | ExclusiveLock | t | t virtualxid | | | | | 4/13 | | | | | 4/13 | 1787 | ExclusiveLock | t | t advisory | 16384 | | | | | | 0 | 1 | 1 | 5/4 | 1789 | ExclusiveLock | t | f advisory | 16384 | | | | | | 0 | 1 | 1 | 4/13 | 1787 | ExclusiveLock | f | f (4 rows) Time: 3.748 ms ``` 通过查询pg_locks,可以看到pid 1789持有advisory lock(granted = t),而pid 1787在等待该锁(granted = f). **参考资料** [Postgres Locking Revealed](https://engineering.nordeus.com/postgres-locking-revealed/)