[Georuby-devel] Aggregate Envelopes in MySQL

[Chris Ingrassia]

	I think I've mentioned this before, but I was looking for a way to  
get an aggregate Envelope for a collection of geometries from a MySQL  
table.  I haven't really had any luck finding a built-in way to do  
that, but it was holding me up with something I needed to get done,  
so I came up with a way to hack it, although I think it's really ugly.

	Basically you have to extract the points from the exterior ring of  
the polygon envelope geometries returned from MySQL and do MIN([X|Y) 
(...)) on them.

	Here's a snippet from one of my models with an example ('geometry'  
is the name of the relevant geometry column):

def minx
     calculate(:min, :all, :select => 'X(StartPoint(ExteriorRing 
(Envelope(geometry))))').to_f
end

def maxy
     calculate(:max, :all, :select => 'Y(PointN(ExteriorRing(Envelope 
(geometry)), 3))').to_f
end

I find that solution to be really, really ugly, but I don't think I'm  
going to do much better. I end up combining the min/max x and y  
values into a polygon later on.

I was hoping that somebody out there might see that and might see  
something that I didn't and be able to point out a better way of  
doing  this so I wouldn't be afraid to look at my code next week, but  
I'm not holding my breath :)

Couple questions:

1.  If anybody has a PostGIS database handy, can you confirm for me  
that it behaves differently, and that when you do something like:

SELECT Extent(the_geom) from some_spatial_table;

it returns a single row with the aggregate extent?

2. Would having an aggregate extent function built into the mysql  
part of spatial_adapter be useful?  If so, I'll clean up my code and  
can probably get a patch out within the next day or two

Thanks!

-Chris