Re: [PATCH] TODO “Allow LISTEN on patterns” - Mailing list pgsql-hackers

Hi Emanuel,

I did a test over the "UNLISTEN >" behavior, and I'm not sure if this is expected.
This command I assume should free all the listening channels, however, it doesn't
seem to do so:

TODO “Allow LISTEN on patterns” [1] is a bit vague about that feature. So I didn't implement it in the first version of the patch. Also I see that I made a mistake in the documentation and mentioned that it is actually supported. Sorry for the confusion.

Besides obvious reasons I think that your finding is especially attractive for the following reason. We have an UNLISTEN * command. If we replace > with * in the patch (which I actually did in the new version of the patch) then we have a generalisation of the above command. For example, UNLISTEN a* cancels registration on all channels which start with a.

I attached to the email the new version of the patch which supports the requested feature. Instead of > I use * for the reason which I mentioned above. Also I added test cases, changed documentation, etc.

I appreciate your work, Emanuel! If you have any further findings I will be glad to adjust the patch accordingly.

[1] https://www.postgresql.org/message-id/flat/52693FC5.7070507%40gmail.com

Regards,
Alexander Cheshev

Regards,

Alexander Cheshev

On Tue, 9 Jul 2024 at 11:01, Emanuel Calvo <3manuek@gmail.com> wrote:

Hello there,

El vie, 15 mar 2024 a las 9:01, Alexander Cheshev (<alex.cheshev@gmail.com>) escribió:

Hello Hackers,

I have implemented TODO “Allow LISTEN on patterns” [1] and attached
the patch to the email. The patch basically consists of the following
two parts.

1. Support wildcards in LISTEN command

Notification channels can be composed of multiple levels in the form
‘a.b.c’ where ‘a’, ‘b’ and ‘c’ are identifiers.

Listen channels can be composed of multiple levels in the form ‘a.b.c’
where ‘a’, ‘b’ and ‘c’ are identifiers which can contain the following
wildcards:
 *  ‘%’ matches everything until the end of a level. Can only appear
at the end of a level. For example, the notification channels ‘a.b.c’,
‘a.bc.c’ match against the listen channel ‘a.b%.c’.
 * ‘>’ matches everything to the right. Can only appear at the end of
the last level. For example, the notification channels ‘a.b’, ‘a.bc.d’
match against the listen channel ‘a.b>’.

I did a test over the "UNLISTEN >" behavior, and I'm not sure if this is expected.

This command I assume should free all the listening channels, however, it doesn't

seem to do so:

postgres=# LISTEN device1.alerts.%;
LISTEN
postgres=# ;
Asynchronous notification "device1.alerts.temp" with payload "80" received from server process with PID 237.
postgres=# UNLISTEN >;
UNLISTEN
postgres=# ; -- Here I send a notification over the same channel
Asynchronous notification "device1.alerts.temp" with payload "80" received from server process with PID 237.

The same happens with "UNLISTEN %;", although I'm not sure if this should have

the same behavior.

It stops listening correctly if I do explicit UNLISTEN (exact channel matching).

I'll be glad to conduct more tests or checks on this.

Cheers,

--

--

Emanuel Calvo

Database Engineering

https://tr3s.ma/aobut