Обсуждение: Question on notifications

Hi,

From the documentation I was able to build a trigger firing upon deletion of a record a function that delivers
tablename_operationas a notification one needs to subscribe to. So in terminal I can say LISTEN persons_delete and
instantlywill receive 

    Asynchronous notification "persons_delete" received from server process with PID 54790.

if there was a delete. But what I don't fully understand is how to do this with PQnotifies. Following the docu I get no
notificationseven though I subscribe to them after successfully connecting to the server the same way I do using
terminal.

Googling didn't give me examples I was able to use. Please, can someone help?

Thanks
Alex

Alexander Reichstadt wrote:
> Thanks, I had checked the example before but couldn't make sense out
of it in terms of wrapping it in
> Objective-C. I left it in C now and it works fine.
>
> The trigger I am using now looks like this:
>
> CREATE FUNCTION notify_trigger() RETURNS trigger AS $$
>
>
> DECLARE
>
>
> BEGIN
> IF ( TG_OP = 'INSERT' ) THEN
> execute 'NOTIFY ' || TG_TABLE_NAME || '_' || TG_OP || ', ' || NEW.oid;
> ELSE
> execute 'NOTIFY ' || TG_TABLE_NAME || '_' || TG_OP || ', ' || OLD.oid;
> END IF;
> return NULL;
> END;
>
>
> $$ LANGUAGE plpgsql;
>
> it works  if I remove transmission of the OID. If I do transmit the
OID I get
>
>
> DETAIL:  The tuple structure of a not-yet-assigned record is
indeterminate.
> CONTEXT:  PL/pgSQL function "notify_trigger" line 1 at EXECUTE
statement
>
>
> The trigger is firing above function AFTER the TG_OP took place. The
error is thrown upon insertion.
> Doesn't the record exist given I trigger AFTER and not BEFORE the
operation took palce?

Right, and I cannot reproduce the error you get.

Here's what I do (on PostgreSQL 9.1.3):

CREATE OR REPLACE FUNCTION notify_trigger() RETURNS trigger
   LANGUAGE plpgsql AS
$$BEGIN
   EXECUTE 'NOTIFY ' || TG_TABLE_NAME || '_' || TG_OP || ', '''
      || CASE WHEN TG_OP = 'INSERT' THEN NEW.oid ELSE OLD.oid END ||
'''';
   RETURN NEW;
END$$;

Observe that the second argument to NOTIFY is a string.

CREATE TABLE t1(val text, PRIMARY KEY (oid)) WITH OIDS;

CREATE TRIGGER t1_trig AFTER INSERT OR UPDATE OR DELETE ON t1
   FOR EACH ROW EXECUTE PROCEDURE notify_trigger();

INSERT INTO t1 (val) VALUES ('test');
UPDATE t1 SET val=NULL;
DELETE FROM t1;

A second session subscribed to the events gets:

Asynchronous notification "t1_insert" with payload "46728" received from
server process with PID 18687.
Asynchronous notification "t1_update" with payload "46728" received from
server process with PID 18687.
Asynchronous notification "t1_delete" with payload "46728" received from
server process with PID 18687.

Yours,
Laurenz Albe

Alexander Reichstadt wrote:
> From the documentation I was able to build a trigger firing upon
deletion of a record a function that
> delivers tablename_operation as a notification one needs to subscribe
to. So in terminal I can say
> LISTEN persons_delete and instantly will receive
>
>     Asynchronous notification "persons_delete" received from server
process with PID 54790.
>
> if there was a delete. But what I don't fully understand is how to do
this with PQnotifies. Following
> the docu I get no notifications even though I subscribe to them after
successfully connecting to the
> server the same way I do using terminal.
>
> Googling didn't give me examples I was able to use. Please, can
someone help?

Did you look at the example in the documentation?
http://www.postgresql.org/docs/current/static/libpq-example.html#LIBPQ-E
XAMPLE-2

Can you post relevant parts of your code?

Yours,
Laurenz Albe