What should return trigger BEFORE DELETE - postgresql

Question

I am trying to make simple trigger function (Postgresql) but I am getting same error 'function did not return any row' in all these cases (just simple examples):
New:
UPDATE somewhere SET something = something - 1;
RETURN NEW;
Old:
UPDATE somewhere SET something = something - 1;
RETURN OLD;
What should I return when I call this function "before delete"? ("after insert/update" works well)
Tyvm for tips!
Full code as requested:
Function:
CREATE OR REPLACE FUNCTION pictogram_frequency_on_delete()
RETURNS trigger AS
$BODY$
DECLARE
new_frequency RECORD;
target_unit RECORD;
current_row RECORD;
units_with_same_type RECORD;
what RECORD;
BEGIN
SET search_path TO 'myScheme';
CASE TG_OP
WHEN 'DELETED' THEN what := OLD;
ELSE what:= OLD;
END CASE;
SELECT unit_type_uid INTO STRICT target_unit
FROM unit
WHERE unit_uid = what.unit_uid;
SELECT count(*) AS exists INTO STRICT current_row
FROM unit_type_pictogram utp
WHERE utp.pictogram_uid = what.pictogram_uid
AND utp.unit_type_uid = target_unit.unit_type_uid;
IF (current_row.exists = 0) THEN
RETURN what; /* return new/old doesnt work too */
END IF;
UPDATE unit_type_pictogram utp
SET frequency = frequency - 1
WHERE utp.pictogram_uid = what.pictogram_uid
AND utp.unit_type_uid = target_unit.unit_type_uid;
RETURN what; /* return new/old doesnt work too */
END
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
Trigger:
CREATE TRIGGER on_delete_frequency
BEFORE DELETE
ON unit_pictogram
FOR EACH ROW
EXECUTE PROCEDURE pictogram_frequency_on_delete();

Answer 1

From documentation:
Trigger functions invoked by per-statement triggers should always
return NULL. Trigger functions invoked by per-row triggers can return
a table row (a value of type HeapTuple) to the calling executor, if
they choose. A row-level trigger fired before an operation has the
following choices:
It can return NULL to skip the operation for the current row. This instructs the executor to not perform the row-level operation that
invoked the trigger (the insertion, modification, or deletion of a
particular table row).
For row-level INSERT and UPDATE triggers only, the returned row becomes the row that will be inserted or will replace the row being
updated. This allows the trigger function to modify the row being
inserted or updated.
A row-level BEFORE trigger that does not intend to cause either of
these behaviors must be careful to return as its result the same row
that was passed in (that is, the NEW row for INSERT and UPDATE
triggers, the OLD row for DELETE triggers).
EDIT
Try something like this:
CREATE OR REPLACE FUNCTION pictogram_frequency_on_delete()
RETURNS trigger AS
$BODY$
BEGIN
UPDATE unit_type_pictogram AS utp
SET frequency = frequency - 1
FROM unit
WHERE utp.pictogram_uid = OLD.pictogram_uid
AND unit_uid = OLD.unit_uid
AND utp.unit_type_uid = unit.unit_type_uid;
RETURN OLD;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;

Answer 2

You should RETURN OLD;.
Your function must be defined as RETURNS trigger.