[postgis-users] Nearest neighbors for a set of points

[Kevin Neufeld]

Depending on how many points are in tables A and B, this cross join is 
the simplest solution that comes to mind to me.

SELECT a.geom, b.geom
FROM a, b
WHERE ST_Distance(a.geom, b.geom) < your-definition-of-near;

That will find all pairs of points (one from table a, one from table b) 
where they are within some definition of near.

If you have a set of points in table A are trying to find a set of 
points in table B, I would collect your points in both tables into 
MULTIPOINT objects.  Thus every "set" is a single geometry so the above 
query works.

You can collect your points into a collection by grouping on some common 
attribute in the set.
i.e.
SELECT set_id, ST_Collect(geom) AS geom
FROM a
GROUP BY set_id;

So, your whole query might be:
SELECT a.set_id, b.set_id
FROM
  (SELECT set_id, ST_Collect(geom) AS geom
   FROM a
   GROUP BY set_id) AS a,
  (SELECT set_id, ST_Collect(geom) AS geom
   FROM b
   GROUP BY set_id) AS b
WHERE ST_Distance(a.geom, b.geom) < 10000;

Cheers,
Kevin

Babu Naidu wrote:
> Hi,
>
> I have a use case where I have to find a set of points near another 
> set of points. For example, given
>
> Set A = { a1, a2, ... an} and Set B = {b1, b2, b3.. bn}
>
> Set A and B are from two different tables. Can I frame a query (SQL) 
> by using postgis functions to find a set of points of type B that are
> near a given set of points of type A?
>
> Any pointers/hints about postgis functions are greatly appreciated.
>
> Thanks
> Babu
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