Re: fixing LISTEN/NOTIFY

Neil, Jim, All:

> Personally, I think delivering all notifications by default is simpler
> behavior for the application programmer to understand. If you want to
> avoid doing work for duplicate notifications, you realistically need to
> implement that yourself anyway.

I agree with Neil; I don't really see duplicate notifications as a problem,
provided that we can deliver a message with the notice. As long as we can
deliver a PK id or other unique information, then we can still use NOTIFY to
do, for example, asynchronous summary counts.

However, I really dislike the idea of blocking on out-of-memory NOTICEs.
While I can see some DBAs wanting that, for most applications I'd want to
implement, I'd rather that the NOTICE were thrown away than have it stop a
query. LISTEN/NOTIFY is a asynchronous messaging mechanism, after all -- the
whole idea is for it *not* to slow down transactions.

Ideally, we'd have three options which could be set administratively (at
startup) in postgresql.conf:
notify_mem = 16384 #in K
notify_mem_full = discard #one of: discard, block, or disk
# discard = simply drop notices over the memory limit
# block = stop queries from committing until notice memory is open
# disk = spill notices to the pg_notice file

You'll notice that I added a spill-to-disk on NOTIFY; I think that should be a
TODO even if we don't implement it immediately.

We should also have a way to detect the situation of notify_mem being full,
both through log notices and by command-line function. Finally, we'd need a
function for the superuser to clear all notices.

I'm thinking here of my personal application for a better LISTEN/NOTIFY: using
pg_memcached to store fast asynchronous materialized views. The idea would
be that you could maintain a materialized view in memory that was no more
than 30 seconds behind, by:
[every 15 seconds]:
check if there are any notices on the view
if so, regenerate the rows indicated in the notify message
if not, next
check if notify_mem is full
if so, regenerate all mattviews from scratch
send clear_notices();

Hopefully people get the idea?

--
Josh Berkus
Aglio Database Solutions
San Francisco