[postgis] distance(geometry,geometry) function

[Frank Warmerdam]

"Timothy H. Keitt" wrote:
> Not in general, no.  This is all made more complex by the fact that
> postgis points and polygons are not seperate SQL types, but are instead
> mixed together within a single column of type "geometry", so you have to
> make your function accept all types of geometry objects.  I'm curious
> why you chose this path instead of making each geometric type a
> different SQL type as with the built-in postgresql geometry types?

Tim,

Dave may want to add to this, but the approach of having a base geometry
type is essentially inherited from the OpenGIS simple features geometry
model.  In particular, for OGC simple features it is necessary to be
able to mix geometry types within a given column in a given table.  

If we implement the full Simple Features for SQL metadata (via auxilary
tables) there will be a mechanism to indicate that particular tables
are restricted to a particular subtype in the total set of possible
geometry types. 

>  Come
> to think of it, I'm not sure what postgis provides that the built-in
> types don't (except maybe much faster queries?).

Holes in polygons, geometry collections and 3d (really 2.5D) support
are the obvious extensions available in the PostGIS geometry model. 

Best regards,

---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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