SAS to PostgreSQL(PADB) code - summing field if they exists - postgresql

Question

I'm having a challenge with a piece of code from SAS that I need to convert to SQL.
Usually I'm very good at this but right not I'm facing a new challenge and so far all my ideas to resolve it are failing and I'm not finding the right way to do so.
I need to be able to pick up field dynamically for this request, like if a field has a certain pattern in it's name I need to sum those fields.
my version of PostgreSQL is 8.0.2, PADB 5.3.3.1 78560
So the table may or may not have a field like bas_txn_03cibc_vcl.
I wrote a function that should output ' ' as bas_txn_03cibc_vcl when the field is not found in the information_schema table and use bas_txn_03cibc_vcl if found.
But when I execute the command I get the error that UDF cannot be used on PADB tables.
"ERROR: XX000: User-defined SQL language function "check_if_field_exists(character varying,character varying,character varying)" cannot be used in a query that references PADB tables."
Right now I'm building a new approach using stored procedure but it will limit the use case. Any other idea on how I can select field dynamically?
Function:
CREATE OR REPLACE FUNCTION check_if_field_exists(_schm text, _tbl text, _field text)
RETURNS text AS
$BODY$
DECLARE
_output_ text:= '' as _field;
BEGIN
EXECUTE 'SELECT column_name into : _output_ FROM rdwaeprd.information_schema.columns
where table_schema='''|| _schm||'''
and table_name='''|| _tbl||'''
and column_name='''|| _field||'''
order by table_name,column_name;';
RETURN _output_;
END
$BODY$
LANGUAGE PLPGSQL;
and then I would use it like this
select indiv_id,ae_psamson.check_if_field_exists('ae_psamson','activ_cc', 'tot_txn_03AMX_AMXE') ,tot_txn_03AMX_AMXD
from activ_cc
group by indiv_id,tot_txn_03AMX_AMXD;
Where the function would either return '' as tot_txn_03AMX_AMXE or simply, tot_txn_03AMX_AMXE.... the idea is to make the query not return an error if the field does not exists.
Like I said I need a new function or approach as this one is not working...

Answer 1

I managed to make a function that make it work!
Basically one of the issue what that information schema was using unsupported function in UDF.
This solution works fine:
CREATE OR REPLACE FUNCTION check_if_field_exists(_schm text, _tbl text, _field text)
RETURNS varchar(55) AS
$BODY$
DECLARE
_output_ varchar(55) :=' 0 as '|| _field;
-- name := (SELECT t.name from test_table t where t.id = x);
BEGIN
EXECUTE 'drop table if exists col_name';
EXECUTE 'create table col_name as SELECT att.attname::character varying(128) AS colname
FROM pg_class cl, pg_namespace ns, pg_attribute att
WHERE cl.relnamespace = ns.oid AND cl.oid = att.attrelid AND ns.nspname='''|| _schm ||'''
and cl.relname='''|| _tbl ||'''
and colname like '''|| _field||''''; -- INTO _output_;
select colname from col_name into _output_ ;
if _output_ is null then
_output_ :=' 0 as '|| _field;
end if;
RETURN _output_ ;
END
$BODY$
LANGUAGE PLPGSQL;