What is the best way to redefine a trigger? (lock issue)

[Marc Mamin <M.Mamin@xxxxxxxxxxxx>]

What is the best way to redefine a trigger? (lock issue)


Hello,
I have deployment/migration scripts that require to be idempotent.

When (re)defining or deleting triggers,  I've lately observed locked statements that seemed never to release (waited for a few hours).
affected version: PG 10 (and probably PG 12 ?)

My case is similar to that old description and I wonder if the recommendation to first change the trigger function to a no-op function still make sense.

https://stackoverflow.com/questions/24738354/how-to-drop-a-trigger-in-a-resilient-manner-in-postgresql.


In the first observed case, with a test db, I did kill all  existing connections to the db and tried to drop the trigger with a fresh new connection.
This again resulted in a long lasting lock and I gave up, tipping on a db corruption.

What does happen in the background, that can make a trigger deletion fail? 
Are there situation where row level locks instead of table level locks are acquired?
Coul background processeslike vacuumplay a role here?

As I've observed this problem only a very few times, I guess it is not easily reproducable.

attached is an picture of pg_stat_activity during such a lock,

thanks,
Marc Mamin

here an example of a such a deployment/migration script, all of these scripts are applied  sequentially in separate transactions:
=======================================
SET client_min_messages=error;

CREATE OR REPLACE FUNCTION block_item_cancel()
  RETURNS TRIGGER AS
$BODY$
DECLARE
	blockedItemLevel int;
	client int;

BEGIN
	WITH RECURSIVE rec as
	(
				SELECT s.id as clientref, s."parentRef", a."fruitRef" 
				FROM "ClientDO" s LEFT JOIN "Fruit2ClientDO" a ON (s.id=a."clientRef" and a."fruitRef" = NEW."fruitRef")
				WHERE s.id = (select "clientRef" from "SeenDO" where "id" = NEW."SeenRef")
	UNION ALL
				SELECT s2.id as clientref, s2."parentRef", a2."fruitRef" 
				FROM rec 
				JOIN "ClientDO" s2 on (s2.id=rec."parentRef")
				LEFT JOIN LATERAL (select"fruitRef" from "Fruit2ClientDO" ax WHERE rec."parentRef"=ax."clientRef" and ax."fruitRef" = NEW."fruitRef")  a2 
				   ON TRUE
				WHERE rec."parentRef" IS NOT NULL
				--Only first matching client should be used
				AND rec."fruitRef" IS NULL
	)
	SELECT clientref
	FROM rec 
	WHERE "fruitRef" is not null
	INTO client;

	blockedItemLevel = (NEW."quantitySeened" - NEW."quantityCanceled");
	IF blockedItemLevel > 0 THEN

		UPDATE "BlockedItemAO" SET
				"blockedItem" = blockedItemLevel,
				"modificationDate" = now()
		WHERE "SeenPosRef" = NEW."id";
	ELSE
		DELETE FROM "BlockedItemAO" WHERE "SeenPosRef" = NEW."id";
	END IF;
	RETURN NEW;
END;
$BODY$
  LANGUAGE plpgsql
  COST 100;


DROP TRIGGER IF EXISTS block_item_cancel ON "SeenPosDO";

CREATE TRIGGER block_item_cancel
  AFTER UPDATE OF "quantityCanceled"
  ON "SeenPosDO"
  FOR EACH ROW
  WHEN ( NEW."providerRef" <> 1
         AND 
           (
              NEW."quantityCanceled" IS DISTINCT FROM OLD."quantityCanceled"              
           )
       )
  EXECUTE PROCEDURE block_item_cancel();

Attachment: sessions.PNG
Description: sessions.PNG