public inbox for pgsql-performance@postgresql.org
help / color / mirror / Atom feedFrom: Justin Pryzby <pryzby@telsasoft.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: overestimate on empty table
Date: Sat, 11 Nov 2017 11:19:45 -0600
Message-ID: <20171111171945.GA2167@telsasoft.com> (raw)
In-Reply-To: <9772.1510348781@sss.pgh.pa.us>
References: <20171110204043.GS8563@telsasoft.com>
<9772.1510348781@sss.pgh.pa.us>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-performance>
On Fri, Nov 10, 2017 at 04:19:41PM -0500, Tom Lane wrote:
> Justin Pryzby <pryzby@telsasoft.com> writes:
> > (or, the opposite of the more common problem)
> > As the queued_alters table is typically empty (and autoanalyzed with
> > relpages=0), I see "why":
>
> > ./src/backend/optimizer/util/plancat.c
> > | if (curpages < 10 &&
> > | rel->rd_rel->relpages == 0 &&
> > | !rel->rd_rel->relhassubclass &&
> > | rel->rd_rel->relkind != RELKIND_INDEX)
> > | curpages = 10;
>
> So I'm sure you read the comment above that, too.
> One idea is to say that relpages = reltuples = 0 is only the state that
> prevails for a freshly-created table, and that VACUUM or ANALYZE should
> always set relpages to at least 1 even if the physical size is zero.
> Dunno if that would confuse people.
What about adding && rel->rd_rel->reltuples==0, and make VACUUM/ANALYZE instead
set only reltuples=1, since that's already done at costsize.c: clamp_row_est()
and therefor no additional confusion?
Justin
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: pryzby@telsasoft.com, tgl@sss.pgh.pa.us
Subject: Re: overestimate on empty table
In-Reply-To: <20171111171945.GA2167@telsasoft.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox