事务后运行触发器，而不是每一行 - PostgreSQL 9.4

Question

我有这个触发器来确保所有投票的总和不超过100：

CREATE FUNCTION limit_votes() RETURNS TRIGGER AS $limit_votes$
    DECLARE
        votes_total int;
    BEGIN
        votes_total := NEW.vote + (SELECT SUM(vote) FROM cars WHERE portfolio_id = NEW.portfolio_id AND car != NEW.car);
        IF votes_total > 100 THEN
            RAISE EXCEPTION 'votes_limit_exceeded';
        END IF;
        RETURN NEW;
    END;
$limit_votes$ LANGUAGE plpgsql;
CREATE TRIGGER limit_votes BEFORE INSERT OR UPDATE ON cars FOR EACH ROW EXECUTE PROCEDURE limit_votes();

应该提到“投票”是可以更新的，所以它不仅仅是 INSERT。 这就是我的问题发生的地方，因为一个用户可以更改汽车之间的投票，然后在其他更新减少其他投票之前，触发单行的更新可能会超过 100 的限制。

我希望它有意义。

我希望在 BEGIN ... COMMIT 之后调用触发器，因此我可以在检查一切正常之前更新多行 - 但如果用户尝试更新投票，也会引发异常并且不保存结果超过 100 个。

Answer 1

触发器必须是约束和“之后”触发器。然后你可以使用 DEFERRABLE：

CREATE CONSTRAINT TRIGGER limit_votes
AFTER INSERT OR UPDATE ON cars
DEFERRABLE
FOR EACH ROW EXECUTE PROCEDURE limit_votes();

查看文档：

http://www.postgresql.org/docs/9.1/static/sql-createtrigger.html

Answer 2

根据https://www.postgresql.org/docs/13/sql-createtrigger.html

由于这应该是一个可延迟的约束，并且您希望在事务之后触发过程执行，因此您需要以下内容，

最初推迟

CREATE CONSTRAINT TRIGGER limit_votes
AFTER INSERT OR UPDATE ON cars
DEFERRABLE
INITIALLY DEFERRED 
FOR EACH ROW EXECUTE PROCEDURE limit_votes();

最初立即

如果我们想在每个语句之后触发过程执行。

CREATE CONSTRAINT TRIGGER limit_votes
AFTER INSERT OR UPDATE ON cars
DEFERRABLE
INITIALLY IMMEDIATE 
FOR EACH ROW EXECUTE PROCEDURE limit_votes();

Answer 3

虽然现有答案会处理 run trigger after transaction 部分，但该函数仍将针对每一行运行。为了确保它只运行一次，您可以在额外的 triggers 表上添加中间触发器，如下所示：

-- triggers table for logging and firing a trigger only once
drop table if exists triggers;
create table triggers(
    "tx_id" bigint primary key,
    "name" text,
    "exec_count" int default 0
);

-- the original trigger now fires an insert into the triggers table
create constraint trigger cars_insert_update
after insert or update on cars
deferrable
initially deferred
for each row 
execute function insert_trigger();

-- trigger function to insert a row in the triggers table
create or replace function insert_trigger()
returns trigger as $insert_trigger$
begin
    insert into triggers
    values (txid_current(), tg_name, 0)
    on conflict (tx_id)
    do nothing;
    
    return null;
end
$insert_trigger$ language plpgsql;

-- trigger to run the original trigger function
-- it will only fire once since updates are ignored
drop trigger if exists triggers_insert on triggers;
create constraint trigger triggers_insert
after insert on triggers
deferrable
initially deferred
for each row
when (new."name" = 'cars_insert_update')
execute function limit_votes();