[postgis-devel] KNN and Semantics

[Paul Ramsey]

Correct, for various definitions of the term "reasonable".
P.

On Mon, Sep 26, 2011 at 1:44 PM,  <maplabs at light42.com> wrote:
> What I got out of this so far...
> * only bboxes are available (from the indexes) at execution time, and,
>
> the box of a point is a point.. so point to point distance is a non-issue
>
> * the query geometry is supplied in its entirety
>
> * the idea then is to take a reasonable approach to non-point geometries
> using only their bbox
>
>   -Brian
>
> On Mon, 26 Sep 2011 13:25:14 -0700, Paul Ramsey
>  wrote:
> KNNGist walks the index tree to provide ordered results.
>>
>> Great. But it's walking the tree, so the results have to be based on
>> boxes. In the case of points, the box == the point, so the ambiguity
>> collapses. In the case of everything else, it does not.
>> So, when someone does
>>
>> select * from mytable order by geom <-> 'polygon()'::geometry;
>>
>> what should they get back to provide the minimum surprise? Because
>> they *will* get back results that differ from
>>
>> select * from mytable order by distance(geom, 'polygon()'::geometry);
>>
>> sometimes substantially.
>> P.
>> On Mon, Sep 26, 2011 at 1:19 PM, Paragon Corporation <lr at pcorp.us> wrote:
>> >
>> >> 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?
>> >>
>> >
>> > Not sure I understand your question Paul?   I don't see how nearest
>> > centroids helps much when you are talking
>> > about largish polygons.  I was thinking this would just make the
>> > ST_Expand
>> > like stuff faster? Oh perhaps I misunderstood that. >
>> > Thanks,
>> > Regina
>> >
>> >
>> >
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