Thread

Re: Optimize LISTEN/NOTIFY

Joel Jacobson <joel@compiler.org> — 2025-12-27T12:40:29Z

On Fri, Dec 26, 2025, at 21:12, Joel Jacobson wrote:
> On Tue, Nov 25, 2025, at 21:17, Tom Lane wrote:
>> "Joel Jacobson" <joel@compiler.org> writes:
>>> It looks to me like it would be best with two boolean fields; one
>>> boolean to stage the updates during PreCommit_Notify, that each
>>> pendingActions could flip back and forth, and another boolean that
>>> represents the current value, which we would overwrite with the staged
>>> value during AtCommit_Notify.
>>
>> +1, I had a feeling that a single boolean wouldn't quite do it.
>> (There are various ways we could define the states, but what
>> you say above seems pretty reasonable.)
>
> I've implemented the two boolean approach and think it's good.
>
> The signals arrays are now preallocated during PreCommit_Notify.
>
> More details in the patch message under "Two-phase staging pattern".

New version with some fixes.

I should have mentioned that v31 is based on v28 (v29 and v30 were discarded).

Here is also a write-up of changes from v28 to v31:

0001: No changes.

0002:

* To avoid post-commit OOM hazards, we now allocate hash table entries
  during PreCommit_Notify.  Each listener entry has two boolean flags;
  staged and current.  For each LISTEN/UNLISTEN action the staged flag
  is set/unset during PreCommit_Notify.  The last action's staged value
  per channel is then copied from staged to current during 
  AtCommit_Notify.

* On abort, AtAbort_Notify reverts staged changes.

* The signal arrays are now preallocated during PreCommit_Notify.

* Renamed Exec_UnlistenAllCommit to CleanupListenersOnExit for the
  exit-handler path, since it has different semantics (unconditional
  removal rather than staged/current handling).

In case someone has already started reviewing v31,
these are the changes I made in v32:

0001:

* Added test: Check UNLISTEN * cancels a LISTEN in the same transaction

0002:
* Fixed initialization of QueueBackendStatus fields, corrected the
  LISTEN + UNLISTEN same-transaction case, restructured AtAbort_Notify
  to mirror AtCommit_Notify, and added a guard for OOM during staging.

/Joel