PostgreSQL 11 - using format to assign to variable - sql

Question

I have been awake for well beyond my schedule and I have been stuck with this issue for a long time, I don't even know what I am looking for to solve, but I wish to use format to insert values that I'll be using for column names, and then executing it... but it keeps giving me errors no matter how much I try changing it :c
Heres the part that im trying to do something that doesnt work, but i think you get the idea what im trying to achieve
ratelimit := EXECUTE format('(SELECT %I
FROM users.ratelimits
WHERE user_id = $2)
', $1);
and heres the full code for the brave
CREATE OR REPLACE FUNCTION users.consume_ratelimit(_name text,__user_id integer)
RETURNS boolean
LANGUAGE 'plpgsql'
VOLATILE
PARALLEL UNSAFE
COST 100
AS $BODY$DECLARE
ratelimit INTEGER;
reset_timeout timestamptz;
premium BOOLEAN;
BEGIN
ratelimit := EXECUTE format('(SELECT %I
FROM users.ratelimits
WHERE user_id = $2)
', $1);
reset_timeout := EXECUTE format('(SELECT %I_refresh
FROM users.ratelimits
WHERE user_id = $2)
', $1);
premium := (SELECT users.is_premium($2));
IF premium THEN
RETURN TRUE;
ELSIF reset_timeout <= NOW() THEN
UPDATE users.ratelimits
SET image_refresh = NOW() + '1 hour'::interval,
image = DEFAULT
WHERE user_id = $2;
RAISE NOTICE 'reset';
RETURN TRUE;
ELSE
IF ratelimit > 0 THEN
EXECUTE format('UPDATE users.ratelimits
SET %I = %I - 1
WHERE user_id = $2', $1, $1);
RAISE NOTICE 'decrement';
RETURN TRUE;
ELSIF ratelimit <= 0 THEN
RAISE NOTICE 'out of credits';
RETURN FALSE;
ELSE
EXECUTE format('INSERT INTO users.ratelimits(user_id) VALUES ($2)
ON CONFLICT DO UPDATE SET
%I = excluded.%I,
%I_refresh = excluded.%I_refresh', $1, $1, $1, $1);
RAISE NOTICE 'create';
RETURN TRUE;
END IF;
END IF;
END;$BODY$;

Answer 1

As documented in the manual you need to use into together with EXECUTE to store the result into a variable. This can handle multiple columns/variables as well, so you only need a single EXECUTE to get both values.
For clarity you should reference parameters by name, not by position.
EXECUTE format('SELECT %I, %I_refresh
FROM users.ratelimits WHERE user_id = $1'),
_name, _name)
USING __user_id
INTO ratelimit, reset_timeout;
Note the $1 inside the string for format() is a parameter placeholder used when the SQL statement is executed, and will be replaced with the value of the variable specified in the USING clause.
Variable assignment is also more efficient without a SELECT:
premium := users.is_premium(__user_id);

Answer 2

It seems some_var := EXECUTE ... will not work. Additionally, params inside of an EXECUTE statement are not the same as those in the function - you need to supply them to the execute statement.
I have not tested this, but perhaps
-- q declared above as text
q := format('
SELECT %I
FROM users.ratelimits
WHERE user_id = %s;
', $1, $2);
EXECUTE q INTO ratelimit;
will work? I also removed the parens from the query, those are unnecessary and may the problem themselves.
I have tested the function
CREATE OR REPLACE FUNCTION test_sum (a int, b int)
RETURNS int
AS $$
DECLARE
q text;
result int;
BEGIN
q := FORMAT ('SELECT %s + %s', $1, $2);
EXECUTE q INTO result;
RETURN result;
END
$$ LANGUAGE plpgsql
and that does work. If my suggestion above does not work, perhaps you can use the above as a starting point.