# [postgis-users] Find n Nearest Neighbors for given Point using PostGIS?

Ragi Burhum rburhum at gmail.com
Sat Feb 26 11:32:52 PST 2011

```Another solution is to use a knngist index. It will most likely be
incorporated in a future version of PostGIS. Since it looks like it was
already commited in PostgreSQL trunk, and if you have one of those
brave-trunk-running souls, you should be able to test it with a beta version
of PostgreSQL. You will get a considerable speed gain from using this
approach.

- Ragi

Date: Thu, 24 Feb 2011 20:38:36 -0800 (PST)

> From: Scholle <mstumpp at gmail.com>
> Subject: Re: [postgis-users] Find n Nearest Neighbors for given Point
>        using PostGIS?
> To: postgis-users at postgis.refractions.net
> Message-ID: <31010203.post at talk.nabble.com>
> Content-Type: text/plain; charset=us-ascii
>
>
> Great, didn't consider the geometry/degree difference.... I drastically
> decreased the value for the third parameter of ST_DWithin function and its
> sufficiently fast now...
>
>
>
>
> >
> > Have you tried EXPLAIN to see where the slow part is?
> >
> > But at a guess - consider that st_dwithin uses the geometry unit for it's
> > calculations - so you are searching for everything within 300 degrees
> > (more than halfway around the planet). You may want to try searching a
> > smaller set of data before you sort it to find the closest five.
> >
> > cheers
> >
> > Ben
> >
> > On 25/02/2011, at 12:04 PM, Scholle wrote:
> >
> >>
> >> I am trying to solve the problem of finding the n nearest neighbors
> using
> >> PostGIS:
> >>
> >> Starting Point:
> >>
> >> - Table geoname with geonames (from geonames.org) containing
> >> latitude/longitude (WSG-84)
> >> - Added a GeometryColumn geom with srid=4326 and datatype=POINT
> >> - Filled geom with values: UPDATE geoname SET geom =
> >> ST_SetSRID(ST_Point(longitude,latitude) 4326);
> >> - Created GIST index for geom (CREATE INDEX geom_index ON geoname USING
> >> GIST (geom);) / Clustered geom_index: CLUSTER geom_index ON geoname;)
> >> - Created PRIMARY KEY UNIQUE BTREE index for geonameid
> >>
> >> Problem:
> >> Find n (e.g. 5) nearest neighbors for a given Point in table geoname
> >> represented by id (geoname.geonameid.
> >>
> >> Possible solution:
> >>
> >> Inspired by
> >>
> http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_nearest_neighbor
> ,
> >> I tried the following query:
> >>
> >> "SELECT start.asciiname, ende.asciiname, distance_sphere(start.geom,
> >> ende.geom) as distance " +
> >> "FROM geoname As start, geoname As ende WHERE start.geonameid = 2950159
> >> AND
> >> start.geonameid <> ende.geonameid " +
> >> "AND ST_DWithin(start.geom, ende.geom, 300) order by distance limit 5"
> >>
> >> Processing time: about 60s
> >>
> >> Also tried an approach based on EXPAND:
> >>
> >> "SELECT start.asciiname, ende.asciiname, distance_sphere(start.geom,
> >> ende.geom) as distance " +
> >> "FROM geoname As start, geoname As ende WHERE start.geonameid = 2950159
> >> AND
> >> start.geonameid <> ende.geonameid AND expand(start.geom, 300) &&
> >> ende.geom "
> >> +
> >> "order by distance limit 5"
> >>
> >> Processing time: about 120s
> >>
> >> The intended application is some kind of autocomplete. So, any approach
> >> taking longer than <1s is not applicable. Is it generally possible to
> >> achieve such a response time with PostGIS?
> >> --
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20110226/59e8a81c/attachment.html>
```