由于没有结果目的地，为什么这个函数不能 return 一个整数？

Question

我有一个简单的函数需要 return 来自 select 语句的整数。就是这样：

CREATE OR REPLACE FUNCTION validate(_identityid integer, _postid integer) RETURNS integer
    LANGUAGE plpgsql
AS
$$
BEGIN
    SELECT postid
    FROM post 
    WHERE
            postid = _postid
      AND
            identityid = _identityid;
END;
$$;

如果我运行上面的函数我得到以下错误：

[42601] ERROR: query has no destination for result data Hint: If you want to discard the results of a SELECT, use PERFORM instead.

为什么结果数据没有目的地？为什么不只是 return SELECT 语句？

Answer 1

As documented in the manual查询结果需要存储在某处。

CREATE OR REPLACE FUNCTION validate(_identityid integer, _postid integer) RETURNS integer
    LANGUAGE plpgsql
AS
$$
DECLARE
  l_result integer;
BEGIN
    SELECT postid
      into l_result
    FROM post 
    WHERE
            postid = _postid
      AND
            identityid = _identityid;
  return l_result;
END;
$$;

但是要像那样包装一个简单的 SELECT，无论如何使用 language sql 函数要好得多，您可以在那里直接 return 结果。

CREATE OR REPLACE FUNCTION validate(_identityid integer, _postid integer) 
  RETURNS integer
  LANGUAGE sql
  stable
AS
$$
    SELECT postid
    FROM post 
    WHERE
            postid = _postid
      AND
            identityid = _identityid;
$$;

Answer 2

如果您想使用 plpgsql 而不使用 RETURN 来执行此操作，您需要一个 OUT 参数：

CREATE OR REPLACE FUNCTION public.out_test(in_val integer, OUT out_val integer)
 LANGUAGE plpgsql
AS $function$
BEGIN
    SELECT INTO out_val in_val + 1;
END;
$function$

select out_test(3);
 out_test 
----------
        4

根据文档 Returning:

The return value of a function cannot be left undefined. If control reaches the end of the top-level block of the function without hitting a RETURN statement, a run-time error will occur. This restriction does not apply to functions with output parameters and functions returning void, however. In those cases a RETURN statement is automatically executed if the top-level block finishes.

由于没有结果目的地，为什么这个函数不能 return 一个整数？

Why is this function not able to return an integer due to no destination for result?

postgresql

plpgsql

postgresql-13