sql - 触发错误时回滚事务

Question

我正在尝试检查将要插入系统的房间在那个日期是否已经出租。我已经计算了匹配房间号和日期的行，然后回滚事务。但是我收到以下错误，即使我更改了代码以引发用户定义的异常:

ERROR:  cannot begin/end transactions in PL/pgSQL
HINT: Use a BEGIN block with an EXCEPTION clause instead.
CONTEXT: PL/pgSQL function "checkRoom"() line 17 at SQL statement

CREATE OR REPLACE FUNCTION "checkRoom"() RETURNS TRIGGER AS
$BODY$
DECLARE 
    counter integer;
  BEGIN
    SELECT COUNT("num_sesion")
    FROM "Sesion"
    INTO counter
    WHERE "Room_Name"=NEW."Room_Name" AND "Date"=NEW."Date";

    IF (counter> 0) THEN -- Probably counter>1 as it's triggered after the transaction..
        raise notice 'THERE'S A ROOM ALREADY!!';
        raise exception 'The room is rented at that date';
    END IF;
    RETURN new;
EXCEPTION
    WHEN raise_exception THEN
        ROLLBACK TRANSACTION;
        RETURN new;
END;$BODY$
LANGUAGE plpgsql VOLATILE NOT LEAKPROOF;

然后我创建触发器:

CREATE TRIGGER "roomOcupied" AFTER INSERT OR UPDATE OF "Room_Name", "Date"
ON "Sesion" FOR EACH ROW
EXECUTE PROCEDURE "checkRoom"();

距离我上次接触 SQL 已经 2 年了，plsql 和 plpgsql 之间的变化让我抓狂。

Answer 1

触发函数的几个问题:

• 使用 IF EXISTS (...) THEN而不是计算所有出现的次数。更快，更简单。见:

  • PL/pgSQL checking if a row exists

• 触发函数 AFTER INSERT OR UPDATE 可以只返回 NULL。 RETURN NEW 仅与名为 BEFORE 的触发器相关。 The manual :

  The return value is ignored for row-level triggers fired after an operation, and so they can return NULL.

• 不平衡的单引号。

• 作为@Pavel explained ，您无法从 plpgsql 函数中控制事务。任何未处理的异常都会强制您的整个事务自动回滚。因此，只需删除 EXCEPTION block 。

您的假设触发器已重写:

CREATE OR REPLACE FUNCTION check_room()
  RETURNS TRIGGER AS
$func$
BEGIN
   IF EXISTS (
         SELECT FROM "Sesion"    -- are you sure it's not "Session"?
         WHERE  "Room_Name" = NEW."Room_Name"
         AND    "Date" = NEW."Date") THEN
     RAISE EXCEPTION 'The room is rented at that date';
   END IF;
   RETURN NULL;
END
$func$  LANGUAGE plpgsql;

BEFORE 触发器更有意义。

但是 UNIQUE INDEX ON ("Room_Name", "Date") 会做同样的事情，而且效率更高。然后，任何违反的行都会引发重复键异常并回滚事务(除非被捕获和处理)。在现代 Postgres 中，您可以选择使用 INSERT ... ON CONFLICT ... 跳过或转移此类 INSERT 尝试。见:

• How to UPSERT (MERGE, INSERT ... ON DUPLICATE UPDATE) in PostgreSQL?

高级用法:

• How to use RETURNING with ON CONFLICT in PostgreSQL?