Special query type for Spatial DB

I have read and looked around at what others have done in Spatial DB but there's one type of query which I have not seen people working on yet. If you all have seen this in any research projects or products, please share with us.

In particular, I'm looking for support for queries like: given a bounded region (3D may be), find me n points in a neighborhood that are the closest to each other. Note that there's no base reference point given, except for a bounded region.

Concrete application queries may look like these: a)Given a bounded region in space, find me a cluster of 50 pulsars that are closest to each other. b)Given the state of California, find me a group of 10 fire-stations that are closest to each other.

Please share your references if you have seen anyone working on or have worked on this. Thanks!

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Arthur Yeo