Re: NOTIFY performance

On Fri, Aug 24, 2012 at 1:46 PM, Artur Zając <azajac(at)ang(dot)com(dot)pl> wrote:
> Hello,
>
>
> I would like to create some application using triggers and LISTEN/NOTIFY
> framework. I've tested it, and I noticed that performance of NOTIFY
> significally decreases with increasing number of distinct NOTIFIES in
> transaction.
> I found that function AsyncExistsPendingNotify is responsibe for it. I think
> that complexivity of searching duplicates there is O(N^2). Would be possible
> to improve performance of it? Maybe by using list for elements precedence
> and binary search tree for searching duplicates - with complexivity of
> O(Nlog2(N)).
>
> I'v tested with 50000 of NOTICES. Updating table with 20000 NOTICES when
> searching is not performed took 1,5 second. With searching it took 28
> seconds.

I've confirmed the n^2 behavior on 9.2:
postgres=# select pg_notify(v::text, null) from generate_series(1,10000) v;
Time: 281.000 ms
postgres=# select pg_notify(v::text, null) from generate_series(1,50000) v;
Time: 7148.000 ms

...but i'm curious if you're going about things the right
way...typically I'd imagine you'd write out actionable items to a
table and issue a much broader NOTIFY which taps listeners on the
table to search the action queue. Could you describe your problem in
a little more detail?

merlin