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From: Nicolas Mitchell <[email protected]>
To: hubert depesz lubaczewski <[email protected]>
Cc: [email protected]
Subject: Re: Trigger function
Date: Tue, 27 Jul 2021 21:05:26 +0000
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

On 27 Jul 2021, at 7:38, hubert depesz lubaczewski wrote:

> *after* insert, not *before*.
> But other than that yes.

Apologies for the lengthy reply; I’ve done a lot of thinking today.

I cannot understand why it would be *after* insert: A required column in 
a new host row (host.object) cannot be populated until a new row is 
inserted into the object table. Therefore the function must (surely?) be 
executed prior to an insert (on the host table).

 From the point of view of a user, the command they would issue is:

=> INSERT INTO host (name, domain) VALUES ('gary', 1000001);

I imagined the trigger/function taking the supplied information (name, 
domain) and retrieving the value for host.object before insert.

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
$$
begin
   WITH object_id AS
(INSERT INTO public.object (type)
VALUES (
	( SELECT obtype.id
       FROM public.obtype
       WHERE obtype.name LIKE 'host'
       )
)
RETURNING id)
INSERT INTO host (name, domain, object)
VALUES (NEW.name, NEW.domain, (SELECT * FROM object_id));
RETURN NEW;
end
$$
LANGUAGE 'plpgsql'

CREATE TRIGGER trig__host_bi
   BEFORE INSERT <————————————> or AFTER INSERT
   ON public."host"
   FOR EACH ROW
   EXECUTE PROCEDURE public.func__host__bi();

Later, and this is for another day, after reading your article (which 
mentions view triggers) it occurred to me to try a trigger attached to a 
view of the hosts table. I created a view and attached a trigger (only 
INSTEAD OF allowed, noted) calling the same function:

CREATE VIEW public.v_host
(
   id,
   "name",
   "domain",
   "object"
)
AS
SELECT
   host.id,
   host.name,
   host.domain,
   host.object
FROM host;

CREATE TRIGGER trig__v_host__bi
   INSTEAD OF INSERT
   ON public.v_host
   FOR EACH ROW
   EXECUTE PROCEDURE public.func__host__bi();

CREATE OR REPLACE FUNCTION public.func__host__bi()
RETURNS trigger AS
$$
begin
   WITH object_id AS
(INSERT INTO public.object (type)
VALUES (
	( SELECT obtype.id
       FROM public.obtype
       WHERE obtype.name LIKE 'host'
       )
)
RETURNING id)
INSERT INTO host (name, domain, object)
VALUES (NEW.name, NEW.domain, (SELECT * FROM object_id));

RETURN NEW;
end
$$
LANGUAGE 'plpgsql'

The previous psql command would now be:

=> INSERT INTO v__host (name, domain) VALUES ('gary', 1000001);

Which succeeds when executed,

- inserting a new object (of type host)
- inserting a new host, with the object.id (host.object) of the newly 
created object
- incrementing the two sequences by one each

> If I might suggest:
> https://www.depesz.com/2012/11/14/how-i-learned-to-stop-worrying-and-love-the-triggers/

Thank you, this was really helpful. I expect to be referring back to it 
in the future.

Many thanks,

Nic

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