[arnabdotorg]

Postgres: http://www.postgresql.org/docs/8.1/static/indexes-types.html:

"PostgreSQL provides several index types: B-tree, R-tree, Hash, and GiST. R-tree indexes are suited for queries on two-dimensional spatial data."

PostGIS: http://postgis.refractions.net/ :

"PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS)"

Also see "Programming the K-means Clustering Algorithm in SQL": http://pdf.cx/6gc9a