[postgis-devel] Changing ST_Expand(geometry) to return LINESTRING

Paul Ramsey pramsey at cleverelephant.ca
Thu Feb 26 05:02:01 PST 2015

I’m OK w/ a linestring. The type is just because I need something that is a geometry to hold what I care about, which is the actual box in the header. I think most people understand the “box” idea pretty well, but n-d boxes (particular in the geography case) can be just as confusing in their own right.


Paul Ramsey

On February 26, 2015 at 4:30:52 AM, Sandro Santilli (strk at keybit.net) wrote:

I was looking for a way to expand a 4D point to use the resulting  
"nd-box" in an ND overlap query (&&&) and found the closest function  
to do that being ST_Expand(geometry).  

That function reurns a geometry of type POLYGON, but I'm thinking..  
what's the POLYGON representation for a 3D or 4D box ?  
Having 5 vertices makes it completely arbitrary where to put  
the MIN and the MAX values of each ordinate.  

Wouldn't it be less confusing to return a 2-vertices LINESTRING  
instead ?  

As noted in http://trac.osgeo.org/postgis/ticket/1329  
a 2-vertices LINESTRING might be the closest thing to  
an even less confusing "BOXND" type. It has a direction  
(contrary to a MULTIPOINT) that makes it clear what the  
min and max are (first and last points).  

Or would you prefer to have a new "BOX" geometry type ?  
Or even a new "BOXND" PostgreSQL type ?  

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