触发错误时回滚事务

我正在尝试检查要插入系统的房间在该日期是否已出租。我已经考虑过计算与房间号和日期匹配的行，然后回滚事务。但即使我更改了代码以引发用户定义的异常，我仍收到以下错误：

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 已经过去了 2 年，plsql 和 plpgsql 之间的变化让我抓狂。

触发函数存在几个问题：

• Use IF EXISTS (...) THEN https://stackoverflow.com/questions/11892233/pl-pgsql-checking-if-a-row-exists-select-into-boolean/11892796#11892796而不是计算所有出现的次数。更快、更简单。看：

  • PL/pgSQL 检查行是否存在 https://stackoverflow.com/questions/11892233/pl-pgsql-checking-if-a-row-exists/11892796#11892796

• 有触发功能AFTER INSERT OR UPDATE可以直接返回NULL. RETURN NEW仅与名为的触发器相关BEFORE. 手册 https://www.postgresql.org/docs/current/trigger-definition.html:

  对于操作后触发的行级触发器，返回值将被忽略，因此它们可以返回NULL.

• 单引号不平衡。

• As @Pavel 解释道 https://stackoverflow.com/a/16700934/939860，您无法从 plpgsql 函数内控制事务。任何未处理的异常都会迫使您的整个事务自动回滚。所以，只需删除EXCEPTION block.

你的假设触发器被重写：

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;

A BEFORE触发更有意义。

But a UNIQUE INDEX ON ("Room_Name", "Date")也会做同样的事情，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（除非捕获并处理）。在现代 Postgres 中，你可以选择跳过或转移这样的INSERT尝试与INSERT ... ON CONFLICT .... See:

• 如何在 PostgreSQL 中进行 UPSERT（合并、插入...重复更新）？ https://stackoverflow.com/questions/17267417/how-to-upsert-merge-insert-on-duplicate-update-in-postgresql/17267423#17267423

高级用法：

• 如何在 PostgreSQL 中使用 RETURNING 和 ON CONFLICT？ https://stackoverflow.com/questions/34708509/how-to-use-returning-with-on-conflict-in-postgresql/42217872#42217872