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Re: [HACKERS] Performance testing of COPY (SELECT) TO

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Zoltan Boszormenyi <zboszor(at)dunaweb(dot)hu>
Cc: Andrew Dunstan <andrew(at)dunslane(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-patches(at)postgresql(dot)org
Subject: Re: [HACKERS] Performance testing of COPY (SELECT) TO
Date: 2006-08-28 16:36:30
Message-ID: 20060828163630.GN27526@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Zoltan Boszormenyi wrote:
> Alvaro Herrera írta:

> >Remember that we were talking about supporting views, not tables.  And
> >if a view uses a slow query then you are in immediate danger of having a
> >slow COPY.  This may not be a problem but it needs to be discussed and
> >an agreement must be reached before introducing such a change (and not
> >during feature freeze).
> 
> COPY relname TO meant tables _and_ views to me.
> My previous tsting showed no difference between
> COPY table TO and COPY (SELECT * FROM table) TO.
> Similarly a slow query defined in the view should show
> no difference between COPY view TO and
> COPY (SELECT * FROM view) TO.

The difference is that we are giving a very clear distinction between a
table and a view.  If we don't support the view in the direct COPY, but
instead insist that it be passed via a SELECT query, then the user will
be aware that it may be slow.

"relname" at this point may mean anything -- are you supporting
sequences and toast tables as well?

> And remember, Bruce put the original COPY view TO
> patch into the unapplied queue, without the SELECT
> feature.

All sort of junk enters that queue so that's not an argument.  (Not
meant to insult Bruce -- I'm just saying that he doesn't filter stuff.
We've had patches rejected from the queue before plenty of times.)

> Rewriting COPY view TO internally to
> COPY (SELECT * FROM view) TO is very
> straightforward, even if you think it's ugly.
> BTW, why is it ugly if I call raw_parser()
> from under src/backend/parser/*.c ?
> It is on a query distinct to the query the parser
> is currently running. Or is it the recursion
> that bothers you? It's not a possible infinite
> recursion.

It's ugly because you are forcing the system to parse something that
was already parsed.

On the other hand I don't see why you are arguing in favor of a useless
feature whose coding is dubious; you can have _the same thing_ with nice
code and no discussion.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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