本文簡單介紹了PostgreSQL Locks(Advisory Locks)的基礎知識,原文詳見[Postgres Locking Revealed](https://engineering.nordeus.com/postgres-locking-revealed/),有所刪減和調整.

PostgreSQL把鎖分為三類,table-level, row-level and advisory locks.Table和Row級鎖可以是顯式或隱式鎖,advisory locks通常是顯式鎖.顯式鎖在顯式使用者請求時獲得,而隱式鎖則透過標準SQL命令獲得.

**Advisory Locks**
PG提供了應用自定義的locks,這種locks稱為Advisory Locks -- 系統並沒有強制使用這些鎖,由應用自行正確使用.
有兩種方法請求獲取advisory lock : Session Level和Transaction Level.
session level : 一旦在session level獲取了鎖,除非session終止或者顯式的釋放,否則鎖會一直持有.不同於標準的鎖請求,session level的advisory lock不需要遵守事務語義:在事務期間申請的lock會在rollback後仍會持有,同時就算事務呼叫失敗unlock也是有效的.lock可以多次獲取,對於每一次完整的鎖請求必須在釋放鎖前有相應的unlock.
transaction level : 與普通鎖類似,在事務結束時自動釋放,沒有顯式的unlock操作.
對於同一個Advisory Lock,如果同時請求session leve和transaction level的鎖,那麼會以期望的方式阻塞.
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
```
透過查詢pg_locks,可以看到pid 1789持有advisory lock(granted = t),而pid 1787在等待該鎖(granted = f).

**參考資料**
[Postgres Locking Revealed](https://engineering.nordeus.com/postgres-locking-revealed/)

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

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