From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Joel Jacobson" <joel@compiler.org>
cc: "Chao Li" <li.evan.chao@gmail.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Optimize LISTEN/NOTIFY
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Comments: In-reply-to "Joel Jacobson" <joel@compiler.org>
	message dated "Thu, 16 Oct 2025 22:06:29 +0200"
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Date: Thu, 16 Oct 2025 16:16:48 -0400
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"Joel Jacobson" <joel@compiler.org> writes:
> On Thu, Oct 16, 2025, at 20:16, Joel Jacobson wrote:
>> Building pendingNotifyChannels is O(N^2) yes, but how large N is
>> realistic here?

> 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.

We already de-duplicate identical NOTIFY operations for exactly that
reason (cf. AsyncExistsPendingNotify).  However, non-identical NOTIFYs
obviously can't be merged.

I wonder whether we could adapt that de-duplication logic so that
it produces a list of unique channel names in addition to a list
of unique NOTIFY events.  One way could be a list/hashtable of
channels used, and for each one a list/hashtable of unique payloads,
rather than the existing single-level list/hashtable.

			regards, tom lane