Execute decode function stored in bytea column - postgresql

Question

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!

Answer 1

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
$$DECLARE
v_stmt text;
BEGIN
FOR v_stmt IN
SELECT convert_from(data,'UTF-8')
FROM attachments
LOOP
EXECUTE v_stmt INTO v_result;
RETURN NEXT;
END LOOP;
END;$$;

Answer 2

This is how I have written the function with few changes
CREATE OR REPLACE FUNCTION getmeall(tName text, fNameCol text, dataCol text,fSize
numeric)
RETURNS TABLE(bdata bytea, fname text) LANGUAGE plpgsql AS
$$DECLARE
v_stmt text;
v_name text;
BEGIN
FOR v_stmt,v_name IN
EXECUTE format('SELECT encode(%s, ''escape''), %s FROM %s
WHERE $1 IS NOT NULL AND $2 IS NOT NULL LIMIT $3'
, dataCol, fNameCol, tName)
USING dataCol, fNameCol, fSize
LOOP
fname:=v_name;
IF strpos(v_stmt,'decode') = 1 THEN
EXECUTE 'SELECT ' || v_stmt INTO bdata;
ELSE
bdata:=v_stmt;
END IF;
RETURN NEXT;
END LOOP;
END;$$;
And finally calling it this way.
select * from getmeall('attachments', '"filename"', '"data"',2)