Execute decode function stored in bytea column

I have a bytea column in a table that contains a function decode(). What I have done to get the actual data is as follows:

select filename, convert_from(data,'UTF-8') from attachments limit 20; //this returns me decode function
select decode(E'...','hex'); // I am executing the above returned function

The above is fine as long as I have to select one row. But now my requirement is to get more than one result. How can I get the result in single query? I have tried using pl/pgsql

CREATE OR REPLACE FUNCTION get_data(integer, _type anyelement, OUT _result anyelement)
  AS
$x$
BEGIN

EXECUTE
'SELECT ' || (select convert_from(data,'UTF-8') as data from attachments limit $1)
INTO _result;

END;
$x$
LANGUAGE plpgsql;

But this works only for single row and single column. What I want is a single query to fetch 2 columns without using pl/pgsql if possible. I am using this query from my Java based web app.

Thanks!

Solution

You need procedural code for this, since there is no provision for dynamic statements in SQL.

The following function converts all attachments:

CREATE FUNCTION getemall(
   IN v_type anyelement,
   OUT v_result anyelement
) RETURNS SETOF anyelement
   LANGUAGE plpgsql AS
$$DECLARE
   v_stmt text;
BEGIN
   FOR v_stmt IN
      SELECT convert_from(data,'UTF-8')
      FROM attachments
   LOOP
      EXECUTE v_stmt INTO v_result;
      RETURN NEXT;
   END LOOP;
END;$$;