Another Word For It Patrick Durusau on Topic Maps and Semantic Diversity

September 29, 2011

Indexed Nearest Neighbour Search in PostGIS

Filed under: Geographic Data,Geographic Information Retrieval,PostgreSQL — Patrick Durusau @ 6:36 pm

Indexed Nearest Neighbour Search in PostGIS

From the post:

An always popular question on the PostGIS users mailing list has been “how do I find the N nearest things to this point?”.

To date, the answer has generally been quite convoluted, since PostGIS supports bounding box index searches, and in order to get the N nearest things you need a box large enough to capture at least N things. Which means you need to know how big to make your search box, which is not possible in general.

PostgreSQL has the ability to return ordered information where an index exists, but the ability has been restricted to B-Tree indexes until recently. Thanks to one of our clients, we were able to directly fund PostgreSQL developers Oleg Bartunov and Teodor Sigaev in adding the ability to return sorted results from a GiST index. And since PostGIS indexes use GiST, that means that now we can also return sorted results from our indexes.

This feature (the PostGIS side of it) was funded by Vizzuality, and hopefully it comes in useful in their CartoDB work.

You will need PostgreSQL 9.1 and the PostGIS source code from the repository, but this is what a nearest neighbour search looks like:

PostgreSQL? Isn’t that SQL? 🙂

Indexed nearest neighbour search is a question of results, not ideology.

Better targeting through technology.

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