Question

I have a database full of two-dimensional data – points on a map. Each record has a field of the geometry type. What I need to be able to do is pass a point to a stored procedure which returns the k nearest points (k would also be passed to the sproc, but that’s easy). I’ve found a query at http://blogs.msdn.com/isaac/archive/2008/10/23/nearest-neighbors.aspx which gets the single nearest neighbour, but I can’t figure how to extend it to find the k nearest neighbours.

This is the current query – T is the table, g is the geometry field, @x is the point to search around, Numbers is a table with integers 1 to n:

DECLARE @start FLOAT = 1000; 
WITH NearestPoints AS
(
     SELECT TOP(1) WITH TIES *,  T.g.STDistance(@x) AS dist
     FROM Numbers JOIN T WITH(INDEX(spatial_index)) 
     ON T.g.STDistance(@x) < @start*POWER(2,Numbers.n)
     ORDER BY n
)
SELECT TOP(1) * FROM NearestPoints
ORDER BY n, dist

The inner query selects the nearest non-empty region and the outer query then selects the top result from that region; the outer query can easily be changed to (e.g.) SELECT TOP(20), but if the nearest region only contains one result, you’re stuck with that.

I figure I probably need to recursively search for the first region containing k records, but without using a table variable (which would cause maintenance problems as you have to create the table structure and it’s liable to change – there’re lots of fields), I can’t see how.

Answer 1

What happens if you remove TOP (1) WITH TIES from the inner query, and set the outer query to return the top k rows?

I’d also be interested to know whether this amendment helps at all. It ought to be more efficient than using TOP:

DECLARE @start FLOAT = 1000
        ,@k INT = 20
        ,@p FLOAT = 2;

WITH NearestPoints AS
(
     SELECT *
            ,T.g.STDistance(@x) AS dist
            ,ROW_NUMBER() OVER (ORDER BY T.g.STDistance(@x)) AS rn
     FROM Numbers 
     JOIN T WITH(INDEX(spatial_index)) 
     ON   T.g.STDistance(@x) <  @start*POWER(@p,Numbers.n)
     AND (Numbers.n - 1 = 0 
          OR T.g.STDistance(@x) >= @start*POWER(@p,Numbers.n - 1)
         )
)
SELECT * 
FROM NearestPoints
WHERE rn <= @k;

NB – untested – I don’t have access to SQL 2008 here.