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Subject: Re: Optimize LISTEN/NOTIFY
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: 
 <CAE7r3MJ0VoAJzdLzX0dgfPJpBJxW+wg_pYaCi6mQJi47+qukhg@mail.gmail.com>
Date: Thu, 6 Nov 2025 07:21:17 +0800
Cc: Joel Jacobson <joel@compiler.org>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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To: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/4605CAD6-69D5-4082-B96C-91FC0DE5399D%40gmail.com>
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> On Nov 6, 2025, at 01:51, Arseniy Mukhin =
<arseniy.mukhin.dev@gmail.com> wrote:
>=20
> On Wed, Nov 5, 2025 at 12:22=E2=80=AFPM Chao Li =
<li.evan.chao@gmail.com> wrote:
>>=20
>>=20
>>=20
>>> On Nov 2, 2025, at 04:41, Arseniy Mukhin =
<arseniy.mukhin.dev@gmail.com> wrote:
>>>=20
>>> This condition seems to be redundant. I would say it should always =
be
>>> true, otherwise it would mean that somebody allowed the listener to
>>> skip our notification.
>>=20
>> Hi Arseniy,
>>=20
>=20
> Hi Chao,
>=20
>> Did you read the example I explained in my previous email?
>>=20
>=20
> Yes, I read it. Thank you for the example. It shows the case where we
> can fail to apply 'direct advancement'. I think there are several
> cases where it can happen. IIUC all such cases are about lagging
> listeners that failed to catch up with the head before the notifier
> tries to apply 'direct advancement' to them. Your example is about
> listeners that finished reading but didn't update their positions
> because they were stuck on the lock. I think it is also possible that
> the listener can be in the process of reading or even didn't start
> reading at all (for example listener backend is in the active
> transaction at the moment). In these cases we also can't apply direct
> advancement. Don't know if some of these examples are more important,
> maybe some of them can be met more frequently.

Cool, you got my idea. What I was thinking is to handle both sleeping =
listeners and =E2=80=9Cslow=E2=80=9D listeners. In my view, which =
shouldn=E2=80=99t be too much complicated.

>=20
> I think the current version of 'direct advancement' will work good for
> 'sleepy' listeners, but probably can be not very efficient for
> listeners that get notifications frequently, don't know. But maybe
> it's ok, we have optimization that sometimes works and have a quite
> simple implementation.
>=20

That=E2=80=99s what we don=E2=80=99t know. We now lack a performance =
test for evaluating how =E2=80=9Cdirect advancement=E2=80=9D efficiently =
helps if it only handles sleeping listeners. So what I was suggesting is =
that we should first create some tests, maybe also add a few more =
statistics, so that we can evaluate different solutions. If a simple =
implementation that only handles sleeping listeners would have performed =
good enough, of course we can take it; otherwise we may need to either =
pursue a better solution.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/