Trigger on BEFORE INSERT OR UPDATE fired once or twice? - postgresql

Question

I have created the following trigger;
CREATE TRIGGER material_trigger
BEFORE INSERT OR UPDATE ON materials
FOR EACH ROW
EXECUTE PROCEDURE my_proc ();
I'm struggling to understand when my_proc will be run on an upsert query, such as the below:
--UPSERT
INSERT INTO materials (id, col)
VALUES (1, 1)
ON CONFLICT (id) DO UPDATE SET
col='x'
If I'm running the upsert, am I right to think that:
in the case of an INSERT it will be run once
in the case of an UPDATE it will be run twice
Then logically if I ran this AFTER the upsert I would ensure that the my_proc was triggered only once?

Answer 1

The trigger is, infact fired twice in the case that the UPDATE is carried out. This can be demonstrated using the follwing:
-- create table
CREATE TABLE IF NOT EXISTS example (uuid int unique);
-- create function
CREATE OR REPLACE FUNCTION print_function ()
RETURNS TRIGGER
AS $body$
BEGIN
RAISE NOTICE 'Starting';
RAISE NOTICE 'METHOD: %' , TG_OP;
RETURN NEW;
END;
$body$
LANGUAGE plpgsql;
-- create trigger function
CREATE TRIGGER example_trigger
BEFORE INSERT OR UPDATE ON example
FOR EACH ROW
EXECUTE PROCEDURE print_function ();
-- upserts
INSERT INTO example(uuid) VALUES (1) ON CONFLICT(uuid) DO UPDATE SET uuid=5;
INSERT INTO example(uuid) VALUES (1) ON CONFLICT(uuid) DO UPDATE SET uuid=5;
which gives output:
NOTICE: Starting
NOTICE: METHOD: INSERT -- Populated with inital value
NOTICE: Starting
NOTICE: METHOD: INSERT -- Fired a first time for insert (can't insert as exists)
NOTICE: Starting
NOTICE: METHOD: UPDATE -- Fired a second time for update
INSERT 0 1