Re: Notifications within triggers seem to compromise performance

Le 28/10/2019 à 15:22, Tom Lane a écrit :
> =?UTF-8?Q?Gr=c3=a9goire_de_Turckheim?= <gdeturckheim(at)scaleway(dot)com> writes:
>> I guess we stumbled upon a performance issue with notifications sent within triggers (using PostgreSQL version 11.5)
>> and I'd like your opinion about this.
> We made some performance improvements for NOTIFY just a couple months
> ago, cf commits b10f40bf0, bb5ae8f6c, bca6e6435, 51004c717. It would
> be interesting to know how much those changes helped your use-case.
Thanks for your quick reply!

If my understanding of the problem is correct, there is no performance
issue with the notification itself.
The problem is the following: a system-wide lock is taken pre-commit, so
any other transaction with a NOTIFY will have to wait for other
transactions to complete before it can leave its own pre-commit stage.
Is this wording better to clarify my explanation attempt ? :)

In my case, ~90% of the data is in tables with triggered notifications,
all of this data updates become "single threaded", whatever the table it
is in.
>
> I'm quite disinclined to reduce the correctness guarantees around
> NOTIFY for performance's sake. That's the sort of thing that sounds
> like a good idea until you find out that it subtly breaks your
> application, and then you've got nothing.
100% agreed, this is why my suggestion was to make it an option. From a
user perspective, it seems very complex to understand if this option is
to be used or not. I really don't know how to present such an option to
the user.

There also may be better ways to do it, I suggested different queues
(and thus locks) for different channels but I have no idea about the
cost of it.

Regards,

--
Grégoire de Turckheim