Loop through table and return a specific column - postgresql

Question

Here is my function:
CREATE OR REPLACE FUNCTION getAllFoo()
RETURNS SETOF "TraderMonthOutstanding" AS
$BODY$
DECLARE
r record;
BEGIN
FOR r IN (select * from "TraderMonthOutstanding"
where "TraderId"=1 and "IsPaid"=false)
LOOP
RETURN NEXT r.TraderId;
END LOOP;
RETURN;
END
$BODY$
LANGUAGE 'plpgsql' ;
I want only TraderId from this loop but I am not able to get it because it gives me an error.
What am I doing wrong?

Answer 1

You declared that getAllFoo() will RETURNS SETOF "TraderMonthOutstanding" (a set of rows with structure of table TraderMonthOutstanding) but then you RETURN NEXT r.TraderId (single field).
You need to either change the declared return type to RETURNS SETOF "TraderMonthOutstanding".TraderId%TYPE or RETURN NEXT r.

Answer 2

You should be returning a setof integer, or a setof TraderMonthOutstanding".TraderId%TYPE.
Or changing perhaps a setof record and returning next r, or event setof TraderMonthOutstanding and declaring r as a TraderMonthOutstanding row type instead of an anonymous record.
Better yet, this seems like the kind of case where you ought to create a view to avoid the overhead of the using a function altogether:
create view getAllFoo as
select *
from "TraderMonthOutstanding"
where "TraderId"=1 and "IsPaid"=false;
select * from getAllFoo;
It's usually terrible practice to loop over a set like that in a function if you're not doing anything useful. (Though I imagine the real one is doing something.)
Lastly, in the event you truly want this in a function, note that you can return query to avoid the loop altogether:
return query
select *
from "TraderMonthOutstanding"
where "TraderId"=1 and "IsPaid"=false;

Answer 3

What #Igor and #Denis said.
If you actually need the loop, which doesn't become apparent from your example, it would work like this:
CREATE OR REPLACE FUNCTION get_all_foo()
RETURNS SETOF "TraderMonthOutstanding"."TraderId"%TYPE AS
$func$
DECLARE
r "TraderMonthOutstanding"; -- use the well known type
BEGIN
FOR r IN
SELECT * FROM "TraderMonthOutstanding"
WHERE "TraderId" = 1
AND "IsPaid" = FALSE -- no parentheses needed
LOOP
-- do whatever necessitates a loop
RETURN NEXT r."TraderId";
END LOOP;
RETURN;
END
$func$ LANGUAGE plpgsql;
Do not quote the language name plpgsql. It's an identifier, not a literal. Current Postgres tolerates it, but it may be weeded out in future versions.
I would advice not to use CaMeL-case identifiers at all in Postgres. Even less if you double-quote some, but not others. That's a LoaDed Footgun. Read the manual about identifiers.
Since you already know the type of the record you are going to use, don't downgrade to an anonymous record, use the well known type to begin with: "TraderMonthOutstanding".
Like you have been told already, the type returned by RETURN has to match what you declared with RETURNS. Since you want only TraderId, adapt RETURNS accordingly. If you want to derive it from the existing table you can use the syntax displayed. Details in the manual. Or you can insert the actual type. Like integer or whatever.
And since you seem to have been using double-quoted CaMeL-case names when creating your table, you have to keep using them that way: double-quoted - even for fields of the derived type in the function body:
RETURN NEXT r."TraderId";
See what I mean with "LoaDed Footgun"?