Find n Nearest Neighbors for given Point using PostGIS?

Question

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 (eg 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 a response time of <1s with PostGIS?

Answer 1

Now since PostGIS 2.0, there's a KNN index for geometry types available. This gives you nearest 5 records with regard to how far they are away from "your location...".

SELECT *
FROM your_table 
ORDER BY your_table.geom <-> "your location..."
LIMIT 5;

See <-> operator in PostgreSQL manual .

Answer 2

As I think you were answered at the list the unit is in degrees so you area almost searching the whole world with 300 degrees in st_dwithin.

If your dataset is that big so you can't work in a projected meterbased projection instead (much faster and less cpu-intensive calculations) you should consider using the geograpphy type instead. Then you can use st_dwithin with meter.

The make things faster you should I would just create a new table with the geometry converted to geography.

But to just test it you can cast on the fly:

SELECT start.asciiname, ende.asciiname, 
ST_Distance(start.geom::geography, ende.geom::geography) as distance 
FROM geoname As start, geoname As ende 
WHERE start.geonameid = 2950159 AND start.geonameid <> ende.geonameid AND
ST_DWithin(start.geom::geography, ende.geom::geography, 300) 
order by distance 
limit 5;

HTH Nicklas