Conditionally chose the set to iterate over in a FOR LOOP - postgresql

Question

I would like to know if I can do a if-elsif or case-when inside a loop. No between the for-loop, but as a statament of the foor-loop to choose between a select or another.
something like this:
I have tried with both, if-elsif and case-when....but none of them worked, and I have been lurking around the net to find something but nope.
CREATE OR REPLACE FUNCTION myfunct(op integer, -vars-)
RETURNS table(-vars-)
LANGUAGE plpgsql
AS $function$
DECLARE
selectop record;
-vars-
BEGIN
FOR selectop in (IF (op=1) THEN
SELECT * FROM mytab WHERE somevar=true;
ELSIF (op=2) THEN
SELECT * from mytab WHERE somevar=false;
END IF;)
-things-
RETURN NEXT;
LOOP
---THINGS---
END;
$function$

Answer 1

I wonder why you don't just set a variable and use that in the query. That seems far easier. But from the academic point of view, it's an interesting question.
The problem is that IF or CASE as a control structure don't return something or evaluate to something. So we cannot use them in such a way.
If we think about returning something, then we may think of functions. So you could place a function call there, that returns a different set depending on an argument. But it seems like the function you want to have should implement exactly that, so this would just shift the problem to another level and ultimately ad infinitum.
So let's think about evaluating to something. Expressions come to our mind and indeed there is CASE as an expression, which we could use to switch. The only thing is, at least as far as I know, that cannot handle sets.
But it can handle arrays. So the idea is to use a CASE expression, that evaluates to (two) different arrays.
We will use the fact, that each table also defines a type in Postgres, that is the type of its rows. So we'll iterate over an array of that type.
The next neat thing in Postgres is, that we can use array_agg() to aggregate the complete table or a subset of it into an array. That's how we'll create the arrays we iterate over.
To iterate over the array we'll use a FOREACH loop. (Yes that's not a FOR loop over a cursor but semantically I'd guess that's close enough.)
Such a function could look like the following. elbat is our table and nmuloc4 that column of it we want to compare a value to, depending on the value of the function's argument switch. The function returns a SETOF elbat, that is a set of records from elbat.
CREATE FUNCTION noitcnuf
(switch integer)
RETURNS SETOF elbat
AS
$$
DECLARE
elbat_array elbat[];
elbat_element elbat;
BEGIN
FOREACH elbat_element IN ARRAY(CASE
WHEN switch = 1 THEN
(SELECT array_agg(elbat)
FROM elbat
WHERE nmuloc4 = true)
WHEN switch = 2 THEN
(SELECT array_agg(elbat)
FROM elbat
WHERE nmuloc4 = false)
ELSE
ARRAY[]::elbat[]
END) LOOP
RETURN NEXT elbat_element;
END LOOP;
RETURN;
END;
$$
LANGUAGE plpgsql;
db<>fiddle
In the ELSE branch of the CASE we just have an empty array of the right type. Otherwise we'd receive an error if we pass an argument to the function for which no branch in the CASE exits. Like that, we just get the empty set in such a case.
Note that this would also work for a view or probably even a set returning function instead of a table (I didn't explicitly test the latter).
But I'd also like to warn, that I suspect this approach to be likely less performant than just building up a query depending on variables and do a classic loop over a cursor or at best reduce it to a set based approach with no loops at all.