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Date: Thu, 16 Oct 2025 22:06:29 +0200
From: "Joel Jacobson" <joel@compiler.org>
To: "Chao Li" <li.evan.chao@gmail.com>
Cc: "Tom Lane" <tgl@sss.pgh.pa.us>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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Subject: Re: Optimize LISTEN/NOTIFY
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On Thu, Oct 16, 2025, at 20:16, Joel Jacobson wrote:
> On Thu, Oct 16, 2025, at 04:54, Chao Li wrote:
>>> On Oct 15, 2025, at 23:36, Joel Jacobson <joel@compiler.org> wrote:
>>> The latest version gets rid of GetPendingNotifyChannels()
>>> and replaces it with the local list pendingNotifyChannels.
>>
>> Sorry for the typo, Yes, I meant to dynahash=E2=80=9D that you have a=
lready=20
>> been using it.
> ...
>> My suggestion of using dynahah was for the same purpose. Because=20
>> list_member_ptr() iterates through all list nodes until find the=20
>> target, so this code is still O(n^2).
>>
>> Using a hash will make it faster. I used to work on project Concourse=20
>> [1]. The system is heavily using the LISTEN/NOTIFY mechanism. There=20
>> would be thousands of channels at runtime. In that case, hash search=20
>> would be much faster than linear search.
>>
>> [1] https://github.com/concourse/concourse
>
> Building pendingNotifyChannels is O(N^2) yes, but how large N is
> realistic here?
>
> Note that pendingNotifyChannels is only the unique channels for the
> notifications in the *current transaction*. At Concourse, did you real=
ly
> do thousands of NOTIFY, with unique channel names, within the same
> transaction?

I tested doing

LISTEN ch1;
LISTEN ch2;
...
LISTEN ch100000;

in one backend, and then

\timing on
BEGIN;
NOTIFY ch1;
NOTIFY ch2;
...
NOTIFY ch100000;
COMMIT;

in another backend.

Timing for the final COMMIT of the 100k NOTIFY:
2.127 ms (master)
1428.441 ms (0002-optimize_listen_notify-v19.patch)

I agree this looks like a real problem, since I guess it's not
completely unthinkable someone might have
some kind of trigger on a table, that could fire off NOTIFY
for each row, possibly causing hundreds of thousands of
notifies in the same db txn.

I tried changing pendingNotifyChannels from a list to dynahash,
which improved the timing, down to 15.169 ms.

Once we have decided which of the three alternatives to go forward with,
I will add the dynahash code for pendingNotifyChannels.

Nice catch, thanks.

/Joel