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From: Chao Li <[email protected]>
To: Arseniy Mukhin <[email protected]>
Cc: Joel Jacobson <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Thu, 6 Nov 2025 07:21:17 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAE7r3MJ0VoAJzdLzX0dgfPJpBJxW+wg_pYaCi6mQJi47+qukhg@mail.gmail.com>
References: <[email protected]>
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> On Nov 6, 2025, at 01:51, Arseniy Mukhin <[email protected]> wrote:
> 
> On Wed, Nov 5, 2025 at 12:22 PM Chao Li <[email protected]> wrote:
>> 
>> 
>> 
>>> On Nov 2, 2025, at 04:41, Arseniy Mukhin <[email protected]> wrote:
>>> 
>>> 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.
>> 
>> Hi Arseniy,
>> 
> 
> Hi Chao,
> 
>> Did you read the example I explained in my previous email?
>> 
> 
> 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 “slow” listeners. In my view, which shouldn’t be too much complicated.

> 
> 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.
> 

That’s what we don’t know. We now lack a performance test for evaluating how “direct advancement” 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/

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