Re: [GENERAL] Potential bug with pg_notify - Mailing list pgsql-general

On 02/13/2017 09:04 AM, François Beaulieu wrote:
>
>> On Feb 13, 2017, at 11:45 AM, Adrian Klaver <adrian.klaver@aklaver.com> wrote:
>>
                                                                      |
>>>
>>>> 3) Are the first row and the second row in the same partition?
>>>
>>> Doubtful, the problem occurs several times a day and we only have one partition a day. Let me check with the above
example.What would be the best way to determine which child a row is in, and the relative position in the child table? 
>>
>> As to position, maybe ctid though it has caveats:
>
> The three rows in my example return a ctid of (742,17), (742,18) and (742,19) respectively, in their child table. So,
probablynot at a partition boundary. 
>
>>> Also; my worker in written in perl and uses DBD::Pg. I haven’t been able to 100% eliminate the module itself as the
causeof the bug. Any suggestions on how I might go about doing that efficiently? 
>>
>> What does the worker do?
>
> Sorry, that's my employer’s classified IP. :-)
> Does it matter?

Only that it makes it harder to give any suggestions on eliminating it
as a source of error if it is a black box.  I don't think, at this
point, it is necessary to see the actual source. If it is possible a
high level synopsis of what it does might be sufficient.

>
>> Could it be the module is not dealing with time zones correctly? Though thinking about this that would seem to
manifesta problem only around the 7th day boundary. So put this down to thinking aloud. 
>
> No, the partitioning scheme seems to be respecting the timezone properly, and my issue is happening every few hours
inthe middle of the day and we’re in UTC+5, so not near the end of the day in UTC. Besides, I believe timestamp without
timezoneassumes the local timezone of the server, which is set to UTC anyway. 
>
> Has the schema eliminated your original theory regarding the delaying of the generation of the _id? I don’t think
thatwould normally be an issue that occurs sporadically and the _id seems to be part of the INSERT, which would
indicatethat, as it should, it’s done generating before my trigger is called. 

I don't see anything that would explain a delay. Still the fact remains
that in most cases the notify captures the _id, but in some cases it
does not. Going back to your OP I realized I missed that the
NEW.userfield was also not coming through. So that seems to confirm that
pg_notify() is firing before it gets access to NEW.*. Having said that I
have no idea why?

The only thing I can think to do is(untested):

CREATE OR REPLACE FUNCTION notify_trigger() RETURNS trigger AS $$
DECLARE
    _newid integer;
BEGIN
   SELECT NEW._id INTO _newid;
   IF _newid IS NULL OR NOT FOUND THEN
    RAISE NOTICE 'NEW._id is NULL/NOT FOUND';
    pg_sleep(0.1); --Or whatever interval you want.
   END IF;
   PERFORM pg_notify('watchers', TG_TABLE_NAME || ',' || NEW._id|| ','
|| NEW.userfield);
   RETURN new;
END;
$$ LANGUAGE plpgsql;

Not really a solution but it might help determine whether it is a timing
issue. Also this is probably something that should be done on a test
server to be safe.


>
> Thanks,
> -=François Beaulieu
> SBK Telecom
>


--
Adrian Klaver
adrian.klaver@aklaver.com