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? > -- > View this message in context: > http://old.nabble.com/Find-n-Nearest-Neighbors-for-given-Point-using-PostGIS--tp31010122p31010122.html > Sent from the PostGIS - User mailing list archive at Nabble.com. > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users