Pass a Table Column Name in a PostgreSQL Function - postgresql

Question

I've read some posts about using table column names in a PostgreSQL function but I couldn't make it work for me.
I have this simple function
DROP FUNCTION IF EXISTS public.benchmark(CHARACTER VARYING, CHARACTER VARYING, BIGINT, BIGINT, BIGINT);
CREATE OR REPLACE FUNCTION benchmark(params CHARACTER VARYING, colName CHARACTER VARYING, idFrom BIGINT, idTo BIGINT, testNumber BIGINT) RETURNS SETOF RECORD AS $$
DECLARE
elemArray TEXT[] := ARRAY(SELECT colName FROM public.test WHERE test.id BETWEEN idFrom AND idTo);
selectedElem RECORD;
elem TEXT;
BEGIN
FOREACH elem IN ARRAY elemArray LOOP
raise notice 'elem Value: %', elem;
SELECT elem INTO selectedElem;
RETURN NEXT selectedElem;
END LOOP;
END;
$$ LANGUAGE plpgsql;
When I execute it with
SELECT * FROM public.benchmark('ad','name',1,2,1) AS x(Item TEXT);
I get
and what I should be getting are the name column values between idFrom and idTo. How can I use the colName variable as an actual column name in elemArray TEXT[] := ARRAY(SELECT colName FROM public.test WHERE test.id BETWEEN idFrom AND idTo);

Answer 1

You may use RETURNS TABLE + RETURN QUERY EXECUTE for dynamic columns.
CREATE OR REPLACE FUNCTION benchmark(params CHARACTER VARYING, colName
CHARACTER VARYING, idFrom BIGINT, idTo BIGINT, testNumber BIGINT)
RETURNS TABLE (colvalue TEXT) AS
$$
BEGIN
RETURN QUERY EXECUTE --dynamic query
format('SELECT %I::TEXT FROM test WHERE test.id BETWEEN $1 AND $2',colName)
--dynamic cols --bind parameters
USING idFrom,idTo;
END;
$$ LANGUAGE plpgsql;
Demo
EDIT
I just want to populate it with the elements of colName and use the
array later in the extended code
You could use ARRAY_AGG & load it into an array variable instead.
CREATE OR REPLACE FUNCTION benchmark(params CHARACTER VARYING, colName
CHARACTER VARYING, idFrom BIGINT, idTo BIGINT, testNumber BIGINT)
RETURNS void AS
$$
DECLARE
elemArray TEXT[];
elem TEXT;
BEGIN
EXECUTE format('SELECT array_agg(%I::TEXT) FROM test
WHERE test.id BETWEEN $1 AND $2',colName)
USING idFrom,idTo INTO elemArray ;
FOREACH elem IN ARRAY elemArray LOOP
raise notice 'elem Value: %', elem;
END LOOP;
END;
$$ LANGUAGE plpgsql;
knayak=# DO $$
knayak$# BEGIN
knayak$# PERFORM benchmark('ad','name',1,2,1);
knayak$# END
knayak$# $$;
NOTICE: elem Value: TalG
NOTICE: elem Value: John Doe
DO