Обсуждение: Sending Results From One Function As Input into Another Function

Greetings,

I need to send the results (SETOF RECORDS) from one function into another
function, to produce another result (SETOF RECORDS). I am not quite sure how
to do get this done. The first function filters a large table down a more
manageable dataset. I want to send the results of this first function to
another function, where computations are performed. I could combine into a
single function, but I would lose some flexibility that I would like to
maintain by keeping the two functions separate. Preliminary research
suggests that cursors might be the way to go, but I am not too experienced
with the use of cursors and was unable to find good examples. Any help would
be greatly appreciated...

Jeff

Jeff Adams wrote:
> I need to send the results (SETOF RECORDS) from one function into
another
> function, to produce another result (SETOF RECORDS). I am not quite
sure how
> to do get this done. The first function filters a large table down a
more
> manageable dataset. I want to send the results of this first function
to
> another function, where computations are performed. I could combine
into a
> single function, but I would lose some flexibility that I would like
to
> maintain by keeping the two functions separate. Preliminary research
> suggests that cursors might be the way to go, but I am not too
experienced
> with the use of cursors and was unable to find good examples. Any help
would
> be greatly appreciated...

Here's an example:

SELECT * FROM test;

 id |  val
----+-------
  1 | one
  2 | two
  3 | three
  4 | four
(4 rows)

CREATE FUNCTION filter() RETURNS refcursor
   LANGUAGE plpgsql STABLE STRICT AS
$$DECLARE
   /* assignment gives the cursor a name */
   curs refcursor := 'curs';
BEGIN
   OPEN curs FOR
      SELECT id, val FROM test WHERE id%2=0;
   RETURN curs;
END;$$;

CREATE FUNCTION compute(curs refcursor) RETURNS text
   LANGUAGE plpgsql STABLE STRICT AS
$$DECLARE
   v test;  -- row type for table
   r text := '';
BEGIN
   LOOP
      FETCH curs INTO v;
      EXIT WHEN v IS NULL;
      r := r || v.val;
   END LOOP;
   RETURN r;
END;$$;

SELECT compute(filter());

 compute
---------
 twofour
(1 row)

Yours,
Laurenz Albe

On Mon, Sep 26, 2011 at 6:49 AM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
> Jeff Adams wrote:
>> I need to send the results (SETOF RECORDS) from one function into
> another
>> function, to produce another result (SETOF RECORDS). I am not quite
> sure how
>> to do get this done. The first function filters a large table down a
> more
>> manageable dataset. I want to send the results of this first function
> to
>> another function, where computations are performed. I could combine
> into a
>> single function, but I would lose some flexibility that I would like
> to
>> maintain by keeping the two functions separate. Preliminary research
>> suggests that cursors might be the way to go, but I am not too
> experienced
>> with the use of cursors and was unable to find good examples. Any help
> would
>> be greatly appreciated...
>
> Here's an example:
>
> SELECT * FROM test;
>
>  id |  val
> ----+-------
>  1 | one
>  2 | two
>  3 | three
>  4 | four
> (4 rows)
>
> CREATE FUNCTION filter() RETURNS refcursor
>   LANGUAGE plpgsql STABLE STRICT AS
> $$DECLARE
>   /* assignment gives the cursor a name */
>   curs refcursor := 'curs';
> BEGIN
>   OPEN curs FOR
>      SELECT id, val FROM test WHERE id%2=0;
>   RETURN curs;
> END;$$;
>
> CREATE FUNCTION compute(curs refcursor) RETURNS text
>   LANGUAGE plpgsql STABLE STRICT AS
> $$DECLARE
>   v test;  -- row type for table
>   r text := '';
> BEGIN
>   LOOP
>      FETCH curs INTO v;
>      EXIT WHEN v IS NULL;
>      r := r || v.val;
>   END LOOP;
>   RETURN r;
> END;$$;
>
> SELECT compute(filter());
>
>  compute
> ---------
>  twofour
> (1 row)

Another method of doing this which I like to point out is via arrays
of composite types.  It's suitable when the passed sets are relatively
small (say less than 10k) and is more flexible -- forcing all data
manipulation through FETCH is (let's be frank) pretty awkward and with
some clever work you can also involve the client application in a more
regular way.  You can use an implict table type or a specially defined
composite type to convey the data:

create type t as (a int, b text, c timestamptz);

create function filter() returns t[] as
$$
  select array(select row(a,b,c)::t from foo);
$$ language sql;

create function do_stuff(_ts t[]) returns void as
$$
declare
  _t t;
begin
  foreach _t in array _ts
  loop
    raise notice '%', _t;
  end loop;
end;
$$ language plpgsql;

note: foreach in array feature is new to 9.1 -- 8.4+ use unnest() --
before that you have to hand roll unnest().

merlin

Thanks for the response Laurenz. I will give it a go...

Jeff

-----Original Message-----
From: Albe Laurenz [mailto:laurenz.albe@wien.gv.at]
Sent: Monday, September 26, 2011 7:50 AM
To: Jeff Adams *EXTERN*; pgsql-general@postgresql.org
Subject: RE: [GENERAL] Sending Results From One Function As Input into
Another Function

Jeff Adams wrote:
> I need to send the results (SETOF RECORDS) from one function into
another
> function, to produce another result (SETOF RECORDS). I am not quite
sure how
> to do get this done. The first function filters a large table down a
more
> manageable dataset. I want to send the results of this first function
to
> another function, where computations are performed. I could combine
into a
> single function, but I would lose some flexibility that I would like
to
> maintain by keeping the two functions separate. Preliminary research
> suggests that cursors might be the way to go, but I am not too
experienced
> with the use of cursors and was unable to find good examples. Any help
would
> be greatly appreciated...

Here's an example:

SELECT * FROM test;

 id |  val
----+-------
  1 | one
  2 | two
  3 | three
  4 | four
(4 rows)

CREATE FUNCTION filter() RETURNS refcursor
   LANGUAGE plpgsql STABLE STRICT AS
$$DECLARE
   /* assignment gives the cursor a name */
   curs refcursor := 'curs';
BEGIN
   OPEN curs FOR
      SELECT id, val FROM test WHERE id%2=0;
   RETURN curs;
END;$$;

CREATE FUNCTION compute(curs refcursor) RETURNS text
   LANGUAGE plpgsql STABLE STRICT AS
$$DECLARE
   v test;  -- row type for table
   r text := '';
BEGIN
   LOOP
      FETCH curs INTO v;
      EXIT WHEN v IS NULL;
      r := r || v.val;
   END LOOP;
   RETURN r;
END;$$;

SELECT compute(filter());

 compute
---------
 twofour
(1 row)

Yours,
Laurenz Albe