[postgis-devel] KNN and Semantics

[Paul Ramsey]

So, here's the deal, the KNN search works exclusively against the
index. So, it only has boxes available to make decisions (not entirely
true, it actually has the full geometry of the query key, but not of
the index keys). That means it can return an exact answer for
point-on-point queries, but for everything else it'll be a box
approximation. So the n-nearest-boxes.

There are lots of ways to attack the problem... we can do pure
nearest-boxes. We could also convert all the boxes to points, and do
nearest-centroids. This might be easiest to explain, potentially. The
trouble is, we're going to be returning an approximation for
everything except points, so the question is (in my mind) which
approximation is easiest to visualize and work with?

P.