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Date: Thu, 16 Oct 2025 20:16:25 +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 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 al=
ready=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 really
do thousands of NOTIFY, with unique channel names, within the same
transaction?

/Joel