[Qgis-developer] Merging of incompatible changes

[Matthias Kuhn]

On Sat, 2012-10-27 at 21:42 +0200, Martin Dobias wrote:
> On Sat, Oct 27, 2012 at 2:46 PM, Matthias Kuhn <matthias.kuhn at gmx.ch> wrote:
> >
> > Sounds like something worth considering.
> > Something else I have been thinking of and what would increase
> > performance pretty good in some situations would be to be able to select
> > by an expression:
> >
> > myFeatureSet = provider.select( QgsExpression( 'attr1 = 1 AND attr2 > 5'
> > ) )
> > while myFeatureSet.nextFeature(myFeature):
> >   do interesting stuff...
> 
> Yes, this is another feature I have in my mind, something that could
> be later added to QgsFeatureRequest. Actually for expressions there
> are two choices: expressions for QgsExpression and provider's native
> expressions (e.g. SQL supported by PostGIS). The difference is that
> the former ones should work for any provider, while the latter ones
> only for those with some native expression language - those are likely
> to be faster since the expression would be executed in the backend, so
> less data transfer and thus faster fetching.

The former one (native expressions) should be pretty straightforward to
implement and will surely be useful for some very specific tasks and
allows to use the full potential of the backend.

The later one could be inspired by OpenDBX [1] or similar (I have no
personal experience with this. It would be nice if somebody who has
could give advice). I would very much like this approach as it would
allow developers to support all database backends without additional
effort. For the end user there would be a reduced risk for a lock-in
[2].
I think this could be implemented in the data-provider as an abstract
method, which filters all the data locally. Each data provider can then
implement a set of basic expressions supported by the backend to be
evaluated there (and maybe fallback in case of new expressions not yet
implemented in a specific dataprovider)

[1]: http://www.linuxnetworks.de/doc/index.php/OpenDBX
[2]: http://en.wikipedia.org/wiki/Vendor_lock-in

Matthias