postgres: function with select and notify - sql

Question

So, I have a table which describes positions of some entities in space(for simplicity in one dimension, original problem is related )
CREATE TABLE positions(
id VARCHAR (32) UNIQUE NOT NULL,
position VARCHAR (50) NOT NULL
);
And I create a trigger which notifies on every update
CREATE TRIGGER position_trigger AFTER UPDATE ON positions
FOR EACH ROW
EXECUTE PROCEDURE notify_position();
and a function:
CREATE FUNCTION notify_position() RETURNS trigger AS $$
DECLARE
BEGIN
PERFORM pg_notify(
'positions',
TG_TABLE_NAME || ',id,' || NEW.id || ',position,' || NEW.position
);
RETURN new;
END;
$$ LANGUAGE plpgsql;
how can I change the function to notify only when there are few entities in the same position.
e.g. consider a table after update
id |positions
-------------
id1 |10
id2 |10
id3 |11
I need to call notify with a string 'id1,id2'
Probably, I need to select somehow all the entities which have the same position as updated one and create a list of them(a string which contains comma-separated ids). How can I do that?

Answer 1

From what I understand of your question, you want the following to happen when an entity's position is updated:
If no entities are currently in the new position, do nothing.
Otherwise call notify with a comma-separated list of entity IDs that are currently in the new position.
You can achieve this in your trigger function by using the string_agg aggregation function to construct a list of IDs of matching entities, then checking its length before calling pg_notify:
CREATE FUNCTION notify_position() RETURNS trigger AS $$
DECLARE
colocated_ids TEXT;
BEGIN
-- Construct list of IDs of co-located entities.
colocated_ids := (
SELECT string_agg(id, ',')
FROM positions
WHERE position = NEW.position AND id != NEW.id
);
-- Notify listeners if some co-located entities were found.
IF length(colocated_ids) > 0 THEN
PERFORM pg_notify('positions', colocated_ids);
END IF;
RETURN new;
END;
$$ LANGUAGE plpgsql;
Note that without the AND id != NEW.id check, the ID of the updated entity will also appear in the list. You could avoid this by making this trigger fire BEFORE UPDATE instead of AFTER UPDATE.