Subject: Re: multivariate statistics v14
To: Jeff Janes <jeff.janes@gmail.com>
References: <1457906378.27231.10.camel@2ndquadrant.com>
 <20160316.173111.82683171.horiguchi.kyotaro@lab.ntt.co.jp>
 <7aa6145f-302b-2077-b961-702cc61710bb@2ndquadrant.com>
 <20160321.080001.265512344372645105.t-ishii@sraoss.co.jp>
 <1bc2bd16-8e33-9e22-cdb1-9ddc38fedc4c@2ndquadrant.com>
 <CAMkU=1zBTiYwDc8K=YmL1dmiZqXHPNbeX=svBGJCNTjaGtsiyQ@mail.gmail.com>
Cc: Tatsuo Ishii <ishii@postgresql.org>,
 Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
 Alvaro Herrera <alvherre@2ndquadrant.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <df28c9e7-cb66-02b8-b497-d979e4dabf4f@2ndquadrant.com>
Date: Tue, 22 Mar 2016 10:23:23 +0100
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Hi,

On 03/22/2016 06:53 AM, Jeff Janes wrote:
> On Sun, Mar 20, 2016 at 4:34 PM, Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
>>
>>
>> D'oh. Thanks for reporting. Attached is v16, hopefully fixing the few
>> remaining whitespace issues.
>
> Hi Tomas,
>
> I'm trying out v16 against a common problem, where postgresql thinks
> it is likely top stop early during a "order by (index express) limit
> 1" but it doesn't actually stop early due to cross-column
> correlations.  But the multivariate statistics don't seem to help.  Am
> I doing this wrong, or just expecting too much?

Yes, I think you're expecting a too much from the current patch.

I've been thinking about perhaps addressing cases like this in the 
future, but it requires tracking position within the table somehow (e.g. 
by means of including ctid in the table, or something like that), and 
the current patch does not implement that.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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