lossy index

[Stephen Woodbridge]

Crystal Li wrote:
> Hi All,
>  
> Would any one explain lossy indexes for me? in PostGIS Manual it talked 
> about GiST indexes are assumed to be lossy.Thanks.
>  
> Crystal

I believe that this relates to the fact the gist indexes are based on 
the bbox of the geometry, so that they loose the precise nature of the 
geometry in favor of the faster bbox representation.

So the use of gist indexes is  a && b means that a might interact with b 
and a moer precide test is needed to confirm that. So you see typical 
queries doing:

   where a && b and intersects(a,b)

HTH,
   -Steve