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From: Tom Lane <[email protected]>
To: Joel Jacobson <[email protected]>
Cc: Chao Li <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Thu, 16 Oct 2025 16:16:48 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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"Joel Jacobson" <[email protected]> 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

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