Distance proximity: Use ST_DWithin not Distance

If you have a problem that involves finding the things within X distance of other things or finding what things have nothing within X distance do not use ST_Distance for filtering and also do not try to use ST_Intersects + ST_Buffer. Use ST_DWithin instead. Why?

1. ST_DWithin can use an index and ST_Distance can not

2. ST_Buffer is just an approximation of a buffer and not an exact buffer

Also note that ST_DWithin is supported for both geometry and geography.

Examples

We show examples using geography. Note geometry would use much the same except units are in the spatial reference units.

for geography:

Finding closest things within 1609 meters (~1 mile)

1SELECT roads.road_name, pois.poi_name
2 FROM roads INNER JOIN pois
3   ON ST_DWithin(roads.geog, pois.geog, 1609);

Finding roads with nothing of interest within 1 mile.

We are using the fact that a LEFT JOIN returns null in the left table when no match is found

1SELECT roads.road_name, pois.poi_name
2 FROM roads LEFT JOIN pois
3   ON ST_DWithin(roads.geog, pois.geog, 1609)
4   WHERE pois.gid IS NULL;