nearest neighbor search from xyz coordinates

Hello,

I have a table containing x y z coordinates. But I have no geometry column?
Is it possible to find the k nearest neighbors of any point. Is there
something like a kd-tree Index in postgres???

I know, if i had a geometry column I could make use of the distance
function to find the k nearest neighbors. But my table has millions of
records, thus it would take a very long time to use the distance
function for each point.

Thanks in advance.

Andi