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From: David G. Johnston <[email protected]>
To: Nicolas Mitchell <[email protected]>
Cc: hubert depesz lubaczewski <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Trigger function
Date: Tue, 27 Jul 2021 15:13:22 -0700
Message-ID: <CAKFQuwazmQBDYdt+zcw2WV+3bONRjbe7qoHiMnREYj+VRXEXjA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

On Tuesday, July 27, 2021, Nicolas Mitchell <[email protected]> wrote:

>
> But when I have tried this with the following trigger/function
> (BEFORE/AFTER), PG goes into a loop. The two associated sequences (object,
> host) are incremented until I break the execution but no insert happens in
> either table. My code is causing an endless loop. I’m too green to
> understand why! I’d be grateful for any hints to help me on my way.
>
> CREATE OR REPLACE FUNCTION public.func__host__bi()
> RETURNS trigger AS
> $$
> begin
>
> INSERT INTO host (name, domain, object)
> VALUES (NEW.name, NEW.domain, (SELECT * FROM object_id));
> RETURN NEW;
> end
> $$
> LANGUAGE 'plpgsql'
>
> CREATE TRIGGER trig__host_bi
>   BEFORE INSERT <————————————> or AFTER INSERT
>   ON public."host"
>   FOR EACH ROW
>   EXECUTE PROCEDURE public.func__host__bi();
>

You are getting an infinite cycle because while in the middle of inserting
a row into host, which provokes the trigger, you go and execute another
insert command for host, provoking the same trigger, performing yet another
insert, provoking the same trigger, etc…

When you write a trigger for a table you should be executing commands
against the same table.

You change the data in the ongoing insert by returning a different row from
the trigger function (i.e., modify your “return new;” line - or modify NEW
itself?).

David J.

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