Re: bad COPY performance with NOTIFY in a trigger

On Thu, Feb 4, 2016 at 11:41 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> =?UTF-8?Q?Filip_Rembia=C5=82kowski?= <filip(dot)rembialkowski(at)gmail(dot)com>
> writes:
> > A table has a trigger.
> > The trigger sends a NOTIFY.
> > Test with COPY FROM shows non-linear correlation between number of
> inserted
> > rows and COPY duration.
>
> No surprise, see AsyncExistsPendingNotify. You would have a lot of other
> performance issues with sending hundreds of thousands of distinct notify
> events from one transaction anyway, so I can't get terribly excited about
> this.
>

What kind of issues? Do you mean, problems in postgres or problems in
client?

Is there an additional non-linear cost on COMMIT (extra to the cost I
already showed)?

The 8GB internal queue (referenced in a Note at
http://www.postgresql.org/docs/current/static/sql-notify.html) should be
able to keep ~ 1E8 such notifications (assumed one notification will fit in
80 bytes).

On client side, this seems legit - the LISTENer deamon will collect these
notifications and process them in line.
There might be no LISTENer running at all.

Still, the main problem I get with this approach is quadratic cost on big
insert transactions.
I wonder if this behavior is possible to change in future postgres
versions. And how much programming work does it require.

Is duplicate-elimination a fundamental, non-negotiable requirement?

Thank you,
Filip