Date: Thu, 25 Sep 2014 21:18:00 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Emre Hasegeli <emre@hasegeli.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Heikki Linnakangas <hlinnakangas@vmware.com>
Subject: Re: KNN-GiST with recheck
Message-ID: <20140926011800.GA8130@momjian.us>
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On Sun, Sep 14, 2014 at 11:34:26PM +0400, Alexander Korotkov wrote:
>     > Cost estimation of GiST is a big problem anyway. It doesn't care (and
>     > can't) about amount of recheck for regular operators. In this patch, same
>     > would be for knn recheck. The problem is that touching heap from access

This is very important work.  While our existing KNN-GiST index code
works fine for scalar values and point-to-point distance ordering, it
doesn't work well for 2-dimensional objects because they are only
indexed by their bounding boxes (a rectangle around the object).  The
indexed bounding box can't produce accurate distances to other objects. 

As an example, see this PostGIS blog post showing how to use LIMIT in a
CTE to filter results and then compute the closest object (search for
"LIMIT 50"):

	http://shisaa.jp/postset/postgis-postgresqls-spatial-partner-part-3.html

This patch fixes our code for distances from a point to indexed 2-D
objects.

Does this also fix the identical PostGIS problem or is there something
PostGIS needs to do?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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