Postgis SQL for nearest neighbors

Question

I\'m trying to calculate the nearest neighbors. For that, I need to pass a parameter to limit the maximum distance from the neighbors. For example, which are the nearest neighbo

Answer 1

First, If you are using latitude, longitude, you need to use 4326.

UPDATE season SET geom = ST_PointFromText ('POINT(' || longitude || ' ' || latitude || ')' , 4326 ) ;

Then you create an index on the geom field

CREATE INDEX [indexname] ON [tablename] USING GIST ( [geometryfield] ); 

Then you get the kNN neightbors:

SELECT *,ST_Distance(geom,'SRID=4326;POINT(newLon newLat)'::geometry) 
FROM yourDbTable
ORDER BY
yourDbTable.geom <->'SRID=4326;POINT(newLon newLat)'::geometry
LIMIT 10;

This query will take advantage of kNN functionality of the gist index (http://workshops.boundlessgeo.com/postgis-intro/knn.html).

Still the distance returned will be in degrees, not meters (projection 4326 uses degrees).

To fix this:

SELECT *,ST_Distance(geography(geom),ST_GeographyFromText('POINT(newLon newLat)') 
FROM yourDbTable
ORDER BY
yourDbTable.geom <->'SRID=4326;POINT(newLon newLat)'::geometry
LIMIT 10;

When you calculate the ST_distance use the geography type. There distance is always in meters:

http://workshops.boundlessgeo.com/postgis-intro/geography.html

All this functionality will probably need a recent Postgis version (2.0+). I am not sure though.

Check this for reference https://gis.stackexchange.com/questions/91765/improve-speed-of-postgis-nearest-neighbor-query/