PostgreSQL Function not working with WHERE clause - sql

Question

I am trying to make a function in Postgres to make my queries faster compared to Django ORM. But the problem I am facing is results are coming when there is no WHERE clause in the query.
This is the function and its call which yields 0 rows:
CREATE OR REPLACE FUNCTION public.standard_search(search_term text, similarity_to integer)
RETURNS TABLE(obj_id integer, app_num integer, app_for text, similarity integer)
LANGUAGE 'plpgsql'
AS $BODY$
DECLARE
similarity integer;
BEGIN
RETURN QUERY
SELECT mark.id, mark.app_num, mark.app_for::text, levenshtein($1, focusword.word) AS similarity
FROM mark
INNER JOIN focusword ON (mark.id = focusword.mark_id)
WHERE similarity <= $2
ORDER BY similarity, mark.app_for, mark.app_num;
END
$BODY$;
select * from public.standard_search('millennium', 4)
This is the function and its call which is giving me results but is slow as the filtering is done in the function call:
CREATE OR REPLACE FUNCTION public.standard_search(search_term text, similarity_to integer)
RETURNS TABLE(obj_id integer, app_num integer, app_for text, similarity integer)
LANGUAGE 'plpgsql'
AS $BODY$
DECLARE
similarity integer;
BEGIN
RETURN QUERY
SELECT mark.id, mark.app_num, mark.app_for::text, levenshtein($1, focusword.word) AS similarity
FROM mark
INNER JOIN focusword ON (mark.id = focusword.trademark_id)
ORDER BY similarity, trad.app_for, mark.app_num;
END
$BODY$;
select * from public.standard_search('millennium', 4) where similarity <= 4
Can anyone shed some light on what is actually going wrong here? After this I can work on the performance improvements.
I was unable to perform this via VIEWS as it required at least one parameter, i.e., search_term to be passed into the levenshtein() function.
I was also facing a problem of passing a tuple as a parameter in the function which is again to be used in the where clause like:
WHERE mark.class in (1,2,3,4,5)
I was doing this previously via RawSQL feature of Django ORM, but trying to do it here because of the performance improvement gains.

Answer 1

The identifier similarity is used in a confusing variety of ways. Not all make sense ...
CREATE OR REPLACE FUNCTION public.standard_search(search_term text, similarity_to integer)
RETURNS TABLE(obj_id integer, app_num integer, app_for text, similarity integer) -- ①
LANGUAGE 'plpgsql'
AS $BODY$
DECLARE
similarity integer; -- ②
BEGIN
RETURN QUERY
SELECT mark.id, mark.app_num, mark.app_for::text, levenshtein($1, focusword.word) AS similarity -- ③
FROM mark
INNER JOIN focusword ON (mark.id = focusword.mark_id)
WHERE similarity <= $2 -- ④
ORDER BY similarity, mark.app_for, mark.app_num; -- ⑤
END
$BODY$;
① ... as column in the return type defined by RETURNS TABLE - effectively an OUT parameter.
② ... as variable - overruling visibility of the OUT parameter. But why?
③ ... as column alias in the SELECT list.
④ ... in the WHERE clause, which makes no sense. It does not refer to ③ (like you seem to assume). Output names are not visible in the WHERE clause. There you can only refer to input column names - or variables and parameters. Since the variable (hiding the parameter, but that makes no difference here) is NULL, no rows are returned. Ever.
⑤ ... in ORDER BY, which resolves to the column alias defined in ③
This is madness. Even I had a hard time figuring out which is visible where, and I have some experience with this. Avoid naming conflicts like this. Follow some naming convention and use distinct names for parameters, variables, column names and aliases!
Related:
PostgreSQL does not accept column alias in WHERE clause
Postgres Function NULL value for row that references NEW
How to return result of a SELECT inside a function in PostgreSQL?
Possible solution
This would make more sense:
CREATE OR REPLACE FUNCTION public.standard_search(_search_term text, _similarity_to integer)
RETURNS TABLE(obj_id integer, app_num integer, app_for text, similarity integer) AS
$func$
SELECT m.id, m.app_num, m.app_for::text, levenshtein(_search_term, f.word) AS sim
FROM mark m
JOIN focusword f ON m.id = f.mark_id
WHERE levenshtein($1, f.word) <= _similarity_to
ORDER BY sim, m.app_for, m.app_num;
$func$ LANGUAGE sql;