Date: Wed, 14 Aug 2019 17:42:36 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Etsuro Fujita <etsuro.fujita@gmail.com>
Cc: Tatsuro Yamada <tatsuro.yamada.tf@nttcom.co.jp>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: progress report for ANALYZE
Message-ID: <20190814214236.GA12940@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: 
 <CAPmGK15VkpMALmhOF9QEm=DS_AQmj36V26ZZQbzBo5_p8igG8w@mail.gmail.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Precedence: bulk

On 2019-Aug-14, Etsuro Fujita wrote:

> On Tue, Aug 13, 2019 at 11:01 PM Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
> > On the subject of FDW support: I did look into supporting that before
> > submitting this.  I think it's not academically difficult: just have the
> > FDW's acquire_sample_rows callback invoke the update_param functions
> > once in a while.  Sadly, in practical terms it looks like postgres_fdw
> > is quite stupid about ANALYZE (it scans the whole table??) so doing
> > something that's actually useful may not be so easy.  At least, we know
> > the total relation size and maybe we can add the ctid column to the
> > cursor in postgresAcquireSampleRowsFunc so that we have a current block
> > number to report (becing careful about synchronized seqscans).
> 
> I don't follow this thread fully, so I might miss something, but I
> don't think that's fully applicable, because foreign tables managed by
> postgres_fdw can be eg, views on the remote side.

Oh, hmm, well I guess that covers the tables and matviews then ... I'm
not sure there's a good way to cover foreign tables that are views or
other stuff.  Maybe that'll have to do.  But at least it covers more
cases than none.

> > I do wonder why doesn't postgres_fdw use TABLESAMPLE.
> 
> Yeah, that's really what I'm thinking for PG13; but I think we would
> still need to scan the whole table in some cases (eg, when the foreign
> table is a view on the remote side), because the TABLESAMLE clause can
> only be applied to regular tables and materialized views.

Sure.

> > I did not look at other FDWs at all, mind.
> 
> IIUC, oracle_fdw already uses the SAMPLE BLOCK clause for that.  Right?

Yeah, it does that, I checked precisely oracle_fdw this morning.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services