函数中的Postgres SELECT ... FOR UPDATE [英] Postgres SELECT ... FOR UPDATE in functions

问题描述

关于使用Post gres函数中的SELECT…FOR UPDATE行级锁定，我有两个问题：

I have two questions about using SELECT … FOR UPDATE row-level locking in a Postgres function:

• 我选择哪几列？它们与我需要锁定然后更新的数据有什么关系吗？

• Does it matter which columns I select? Do they have any relation to what data I need to lock and then update?

SELECT * FROM table WHERE x=y FOR UPDATE;

vs

SELECT 1 FROM table WHERE x=y FOR UPDATE;

在不将数据保存到某处的情况下，我无法在函数中进行选择，所以我保存到虚拟变量。这似乎很老套；

I can't do a select in a function without saving the data somewhere, so I save to a dummy variable. This seems hacky; is it the right way to do things?

这是我的功能：

CREATE OR REPLACE FUNCTION update_message(v_1 INTEGER, v_timestamp INTEGER, v_version INTEGER)
RETURNS void AS $$
DECLARE
    v_timestamp_conv TIMESTAMP;
    dummy INTEGER;
BEGIN
    SELECT timestamp 'epoch' + v_timestamp * interval '1 second' INTO v_timestamp_conv;
    SELECT 1 INTO dummy FROM my_table WHERE userid=v_1 LIMIT 1 FOR UPDATE;
    UPDATE my_table SET (timestamp) = (v_timestamp_conv) WHERE userid=v_1 AND version < v_version;
END;
$$  LANGUAGE plpgsql;

推荐答案

哪个重要我选择的列？

Does it matter which columns I select?

不，没关系。即使使用从表WHERE ... FOR UPDATE中选择1 ，查询也会锁定满足where条件的所有行。


如果查询从联接中检索行，我们不想锁定联接中涉及的所有表中的行，而只锁定特定表中的行，即 SELECT ... FOR UPDATE OF-OF-TABLENAMES 语法可能有用：

http://www.postgresql.org/docs/9.0/static/sql-select.html#SQL-FOR-UPDATE-SHARE

No, it doesn't matter. Even if SELECT 1 FROM table WHERE ... FOR UPDATE is used, the query locks all rows that meet where conditions.

If the query retrieves rows from a join, and we don't want to lock rows from all tables involved in the join, but only rows from specific tables, a SELECT ... FOR UPDATE OF list-of-tablenames syntax can be usefull:
http://www.postgresql.org/docs/9.0/static/sql-select.html#SQL-FOR-UPDATE-SHARE

在不将数据保存到某处的情况下，我无法在函数中进行选择，因此我将其保存到虚拟变量。这似乎很老套；

I can't do a select in a function without saving the data somewhere, so I save to a dummy variable. This seems hacky; is it the right way to do things?

在Pl / PgSql中使用 PERFORM 放弃查询结果的命令：

http://www.postgresql.org/docs/9.2/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-SQL-NORESULT



代替：

In Pl/PgSql use a PERFORM command to discard query result:
http://www.postgresql.org/docs/9.2/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-SQL-NORESULT

Instead of:

SELECT 1 INTO dummy FROM my_table WHERE userid=v_1 LIMIT 1 FOR UPDATE;

使用：

PERFORM 1 FROM my_table WHERE userid=v_1 LIMIT 1 FOR UPDATE;

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