[postgis-devel] Issue 137 in postgis: st_distance, shortestline (, st_maxdistance)

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Comment #7 on issue 137 by robe.... at cityofboston.gov: st_distance,  
shortestline (, st_maxdistance)
http://code.google.com/p/postgis/issues/detail?id=137

Nicklas,
I think in those cases, its better to return null rather than an empty  
geometry.  The
main reasons I think its best is that I think that's the way the other  
functions in
PostGIS work when no valid geometry can be given (such as when you  
ST_Simplify
something too much).

The other reason is that if someone is using this to insert/update  a  
geometry
column, then most likely they have a constraint that allows null or a  
linestring or
multilinestring.  If you return empty geometry, they would need to do  
checking to
check if its empty before they can insert -- which adds another level of  
annoyance.

The 3rd reason is that most geos functions don't like GEOMETRY collections  
and barf
when they see one -- so if it is used as a preprocessor for something else  
-- it may
halt execution. (and an empty geometry I think is considered a geometry  
collection --
or hmm maybe we really do support the concept of an empty LINESTRING though  
not sure
what GEOS does with such a thing).  I think Paul may have been working on  
that.

Alternatively I suppose you could return a point (hmm or maybe not since  
you would
still have the annoyance of checking and the function is after all called  
the
shortest_line).

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