To: Marc Cousin <cousinmarc@gmail.com>,
	Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>,
	pgsql-performance@postgresql.org
Subject: Re: inheritance: planning time vs children number vs column number 
In-reply-to: <17604.1298993631@sss.pgh.pa.us> 
References: <201102281038.04945.cousinmarc@gmail.com>
	<201102281947.46590.cousinmarc@gmail.com>
	<4188.1298960419@sss.pgh.pa.us>
	<201103010842.42128.cousinmarc@gmail.com>
	<17604.1298993631@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 01 Mar 2011 10:33:51 -0500"
Date: Tue, 01 Mar 2011 12:33:54 -0500
Message-ID: <22075.1299000834@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

I wrote:
> Marc Cousin <cousinmarc@gmail.com> writes:
>> Yes, for the same test case, with a bit of data in every partition and 
>> statistics up to date, planning time goes from 20 seconds to 125ms for the 600 
>> children/1000 columns case. Which is of course more than acceptable.

> [ scratches head ... ]  Actually, I was expecting the runtime to go up
> not down.  Maybe there's something else strange going on here.

Oh, doh: the failing pg_statistic lookups are all coming from the part
of estimate_rel_size() where it tries to induce a reasonable tuple width
estimate for an empty table (see get_rel_data_width).  Probably not a
case we need to get really tense about.  Of course, you could also argue
that this code is stupid because it's very unlikely that there will be
any pg_statistic entries either.  Maybe we should just have it go
directly to the datatype-based estimate instead of making a boatload
of useless pg_statistic probes.

			regards, tom lane