postgresql - 如何手动锁定和解锁一行？

Question

我试图通过锁定和解锁某些行来避免重复写入。

我有一个表personnel，只要用户尝试编辑一行，我就想锁定该行。

所以

BEGIN;
// I guess I have to lock it somehow here
SELECT * FROM personnel WHERE id = 12;
COMMIT;

一旦完成编辑，我想以相同的样式提交 UPDATE:

BEGIN;
//Unlocking
UPDATE personnel SET ...
WHERE id = 12;
COMMIT;

在此期间，当另一个用户试图编辑同一行时，他会收到一条消息。

Answer 1

锁在事务结束时释放。因此，您需要保持事务打开以维持锁定并在单个事务中完成所有操作:

BEGIN;
SELECT * FROM personnel WHERE id = 12 <b>FOR UPDATE NOWAIT</b>;  -- row level locking
<strike>COMMIT;</strike>

<strike>BEGIN;</strike>
UPDATE personnel SET ...
WHERE id = 12;
COMMIT;

或者，您可以使用限制较少的 FOR SHARE 子句。
Details in the manual on SELECT in the chapter "The Locking Clause".

允许并发读取。引用章节"Row-level Locks" in the manual :

In addition to table-level locks, there are row-level locks, which can be exclusive or shared locks. An exclusive row-level lock on a specific row is automatically acquired when the row is updated or deleted. The lock is held until the transaction commits or rolls back, just like table-level locks. Row-level locks do not affect data querying; they block only writers to the same row.

要让并发尝试立即失败，请使用 NOWAIT 选项 - 对于所有竞争查询。