Postgresql 我可以让 plpgsql 函数在不使用变量的情况下返回整数吗

是的，你可以.有很多方法.

1) RETURN (SELECT ...)

CREATE OR REPLACE FUNCTION get(_param_id integer)
  RETURNS integer AS
$func$
BEGIN
   RETURN (SELECT col1 FROM TABLE WHERE id = _param_id);
END
$func$  LANGUAGE plpgsql;

2) Use an OUT or INOUT parameter

CREATE OR REPLACE FUNCTION get(_param_id integer, OUT _col1 integer)
-- RETURNS integer -- "RETURNS integer" is optional noise in this case
  AS  
$func$
BEGIN
   SELECT INTO _col1  col1 FROM TABLE WHERE id = _param_id;

   -- also valid, but discouraged:
   -- _col1 := col1 FROM TABLE WHERE id = _param_id;
END
$func$  LANGUAGE plpgsql;

More in the manual here.

3) (Ab)use IN parameter

因为Postgres 9.0，你也可以用input parameters作为变量.The release notes for 9.0:

输入参数现在的作用类似于初始化为传入值的局部变量.

CREATE OR REPLACE FUNCTION get(_param_id integer)
  RETURNS integer AS
$func$
BEGIN
   SELECT INTO _param1  col1 FROM TABLE WHERE id = _param1;
   RETURN _param1;

   -- Also possible, but discouraged:
   -- $1 := col1 FROM TABLE WHERE id = $1;
   -- RETURN $1;
END
$func$  LANGUAGE plpgsql;

对于最后一个变量，您可以隐式地输入一个变量，但不必显式地输入它(根据请求).

4) Use a DEFAULT value with an INOUT parameter

这有点特殊.函数体可以为空.

CREATE OR REPLACE FUNCTION get(_param_id integer, INOUT _col1 integer = 123)
  RETURNS integer AS
$func$
BEGIN
   -- You can assign some (other) value to _col1:
   -- SELECT INTO _col1  col1 FROM TABLE WHERE id = _param_id;
   -- If you don't, the DEFAULT 123 will be returned.
END
$func$  LANGUAGE plpgsql;

INOUT _col1 integer = 123是INOUT _col1 integer DEFAULT 123的缩写.更多:

• The forgotten assignment operator "=" and the commonplace ":="

5) Use a plain SQL function instead

CREATE OR REPLACE FUNCTION get(_param_id integer)
  RETURNS integer AS
$func$
   SELECT col1 FROM TABLE WHERE id = _param_id;
   -- use positional reference $1 instead of param name in Postgres 9.1 or older
$func$  LANGUAGE sql;