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

On 10/12/21 12:20 μ.μ., Marc Mamin wrote:
>
> >> 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?
> >A DROP TRIGGER will try to acquire an AccessExclusiveLock on the table, which conflicts with any table level lock (e.g a select acquires an access share lock, so it would cause the DROP TRIGGER to wait.
> >
> >Unfortunately I don't see this in the official docs: https://www.postgresql.org/docs/11/explicit-locking.html .
> >
> >> 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.
>
> >It is very easily reproducible. begin; select .. in one session, begin; drop trigger in a second session.
>
> You can see in the attachment, that the lock exists without any other apparent conflicting session.
It takes two or more to tango. Next time it happens query the pg_locks view, it contains info about locks on objects.
The holding lock is shown as granted, the waiting lock as not granted.
>
> >Do you need to drop/create the trigger or a CREATE OR REPLACE function would suffice?
>
> There are different use cases. Sometimes I only need to drop a trigger or modify its definition (not the function)
Are you using pgbouncer or some other means of suspending traffic into the DB?
>
> >>
> >> 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();
> >>
> >>
> >
> >
> >--
> >Achilleas Mantzios
> >DBA, Analyst, IT Lead
> >IT DEPT
> >Dynacom Tankers Mgmt
> >
> >
> >
> >

--
Achilleas Mantzios
DBA, Analyst, IT Lead
IT DEPT
Dynacom Tankers Mgmt