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/)

ITPub博客

PostgreSQL DBA(76) - Locks(Advisory Locks)：What You Should Know

最新文章