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Subject: Re: Optimize LISTEN/NOTIFY
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: <0a5a20d3-4621-46b3-b2ab-903f63a20dea@app.fastmail.com>
Date: Thu, 16 Oct 2025 10:54:12 +0800
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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To: Joel Jacobson <joel@compiler.org>
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> On Oct 15, 2025, at 23:36, Joel Jacobson <joel@compiler.org> wrote:
>=20
>> I agree with Tom that GetPendingNotifyChannels() is too heavy and =
unnecessary.
>>=20
>> In PreCommit_Notify(), we can maintain a local hash table to record=20=

>> pending nofications=E2=80=99 channel names. dahash also supports hash =
table in=20
>> local memory.
>=20
> I'm confused, I assume you mean "dynahash" since there is no "dahash"
> in the sources? I see dynahash has local-to-a-backend support,
> but I don't see why we would need a hash table for this,
> we just iterate over it once in SignalBackends,
> I think the local list is fine.
>=20
> 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 =
already been using it.

In v18, I see you are building =E2=80=9CpendingNotifyChannels=E2=80=9D =
in PreCommit_Notify() with =E2=80=9CList=E2=80=9D:

```
+		/*
+		 * Build list of unique channels for SignalBackends().
+		 */
+		pendingNotifyChannels =3D NIL;
+		foreach_ptr(Notification, n, pendingNotifies->events)
+		{
+			char	   *channel =3D n->data;
+
+			/* Add if not already in list */
+			if (!list_member_ptr(pendingNotifyChannels, =
channel))
+				pendingNotifyChannels =3D =
lappend(pendingNotifyChannels, channel);
+		}
```

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

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/