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From: Joel Jacobson <[email protected]>
To: Chao Li <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Thu, 16 Oct 2025 20:16:25 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On Thu, Oct 16, 2025, at 04:54, Chao Li wrote:
>> On Oct 15, 2025, at 23:36, Joel Jacobson <[email protected]> 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” that you have already 
> been using it.
...
> My suggestion of using dynahah was for the same purpose. Because 
> list_member_ptr() iterates through all list nodes until find the 
> target, so this code is still O(n^2).
>
> Using a hash will make it faster. I used to work on project Concourse 
> [1]. The system is heavily using the LISTEN/NOTIFY mechanism. There 
> would be thousands of channels at runtime. In that case, hash search 
> 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

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