[postgis-users] Function OVERLAPS() versus &&

[strk at refractions.net]

On Fri, Jan 14, 2005 at 10:04:54AM -0800, James G Wilkinson wrote:
> Here is an example query:
> 
> CREATE TALBE us_pophu_prj AS
>                SELECT a.gid, b.gid AS sec_gid, 
> GEOMUNION(a.the_geom,b.the_geom)
>                FROM us_pophu a, us_tribes b
>                WHERE a.the_geom && b.the_geom AND 
> OVERLAPS(a.the_geom,b.the_geom);
> 
> My questions are as follows:
> 
> (1) Are the use of the '&&' clause and the function OVERLAPS() redundant?

No. && only checks for bounding box overlap, overlaps() performs
topological overlap.

> (2) I know that '&&' will use the GiST index, but does the function 
> OVERLAPS()
> use the GiST index?

No, only operators are allowed to use indexes.

> (3) Is the query I pose the most efficient way to union the two coverages?

You should probably avoid union of a geom with itself ...
AND a.gid != b.gid 

--strk;

> 
> Regards,
> 
> Jim
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