[Qgis-developer] Merging of incompatible changes

[Matthias Kuhn]

On Sat, 2012-10-27 at 11:16 +0200, Sandro Santilli wrote:
> On Sat, Oct 27, 2012 at 01:41:49AM +0200, Tim Sutton wrote:
> 
> > * One of the major limitations I and the many people that contact me
> > about QGIS experience is lack of performance. Just yesterday I got an
> > email from someone in Sudan trying to use QGIS to work with ~300 000
> > point records and it taking 4 hours to do some simple operations on
> > the dataset. 
> 
> I think such cases should be handled by only fetching a sample of the
> data for visualization purpose, rather than the full set. And similarly
> only a subset of vertices could be fetched when facing dense vectors.
> 
> This is something that might need an API change too, as you would need
> the providers to be told when the data has to be fetched only for
> visualization (thus generalization is possible) or for editing (thus
> you want all the data, possibly forcing a restriction on extent).
> 
> Just my 2 cents.
> 
> --strk; 

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...

I'm working here on a dataset with several thousand lines and I want
create a rubberband on some of them (Lets say 500).
Possibilities now are:
 * Request each single feature by id (It's based on a pg view which
takes 0.5 seconds to be built, multiplied by 500: 250seconds...)
 * Request all features, iterate and check if it's in the list. This can
also consume a considerable amount of time depending on the total cound
of features.

I guess what I'm talking about could be called Database Abstraction
Layer.

-- Matthias

(Sorry for the thread hijacking)

> 
>  http://www.cartodb.com - Map, analyze and build applications with your data
> 
>                                        ~~ http://strk.keybit.net 
> 
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