[postgis-devel] ST_Union specification questions

[Pierre Racine]

I would suggest you write SQL tests to test ST_Union rather than writing emails... Would be faster.

I would suggest also that you provided your observation when it is time. Which was one year ago.

I would suggest you write a prototype of your own view of what should be ST_MapAlgebra using your raster iterator. That would be more useful. More constructive. 

Pierre

> -----Original Message-----
> From: postgis-devel-bounces at postgis.refractions.net [mailto:postgis-devel-
> bounces at postgis.refractions.net] On Behalf Of Bryce L Nordgren
> Sent: Monday, November 21, 2011 5:07 PM
> To: PostGIS Development Discussion
> Subject: [postgis-devel] ST_Union specification questions
> 
> So....
> 
> How are the p_, t_ and f_ expressions related? It seems like these accumulate
> values in three different bands? If so, how was the number "three bands"
> determined? If you don't specify one set of expressions, does the corresponding
> band disappear out of the result? Or am I reading it completely wrong: Does
> ST_Union always return a single band, and the "state" expression is some sort of
> "scratch" band which is used to accumulate information, but which is not
> actually returned?
> 
> Also, aggregate functions really should not produce results which depend on the
> order in which the individual records are encountered. Allowing this functionality
> (see specifications: FIRST and LAST set of expressions) is inviting trouble of a
> spurious and random nature, particularly when results from Windows are
> compared with results from linux. The answer should always be the same, just
> like the geometry version of ST_Union always returns the same value. The
> solution may be to limit the functions which are allowed to appear in the
> expression to other aggregates...
> 
> Bryce