Re: Commit fest 2014-12, let's begin!

On Mon, Dec 15, 2014 at 03:58:39PM +0200, Heikki Linnakangas wrote:
> >WITH closest_candidates AS (
> >   SELECT
> >     streets.gid,
> >     streets.name,
> >     streets.geom
> >   FROM
> >     nyc_streets streets
> >   ORDER BY
> >     streets.geom <->
> >     'SRID=26918;POINT(583571.905921312 4506714.34119218)'::geometry
> >   LIMIT 100
> >)
> >SELECT gid, name
> >FROM closest_candidates
> >ORDER BY
> >   ST_Distance(
> >     geom,
> >     'SRID=26918;POINT(583571.905921312 4506714.34119218)'::geometry
> >     )
> >LIMIT 1;
> >
> >See blog posts:
> >http://blog.light42.com/wordpress/?p=102
> >http://workshops.boundlessgeo.com/postgis-intro/knn.html
> 
> Ugh. Ok, sold :-). I'll review...

Just to summarize, the index can only be created on the <-> operator
because that indexes on the center of the bounding box:
http://postgis.net/docs/manual-2.1/geometry_distance_centroid.html

You can't index on ST_Distance() because that computes the minimum
distance between the two objects, which is different for different
points:
http://postgis.net/docs/manual-2.1/ST_Distance.html

I am embarrassed by the LIMIT 100 hack they have to use above to find
the closest polygon to a given point, and this recheck patch is going to
fix that.  I think this will remove the most significant PostGIS wart.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + Everyone has their own god. +