[postgis-devel] ST_Union specification questions

[Bryce L Nordgren]

On Mon, Nov 21, 2011 at 10:56 PM, Pierre Racine <Pierre.Racine at sbf.ulaval.ca
> wrote:

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.
>

A] I was asking about ST_Union.
B] I wasn't talking about the raster iterator at all.
C] More useful and constructive than what? Pointing out something that I
see might cause problems which are potentially very difficult to debug?

Regina's response was constructive, for which I am eternally grateful. In
this case, I learned that postgresql 9+ has a notion of ordered aggregates.
This modifies the question into: Is there any way to require that
expressions which depend on order occur only within an ordered aggregate?
Conversely, how do we disable (and should we attempt to disable) these
expressions on platforms (postgres < 9) which do not support ordered
aggregates, or in contexts where the aggregate ordering is not specified?

At the very least, I think I'd suggest that the documentation for ST_Union
contain a fairly heavyhanded warning about the dangers of using
expressions where the order matters in a non-ordered-aggregate context....

Bryce
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