[Qgis-developer] Performance for selection on attribute table

[Matthias Kuhn]

Hi Régis

The attribute table is not using any providers indexes. And when 
thinking about it a couple of questions arise like stable syntax across 
providers (different providers need different SQL which either needs to 
be translated from QgsExpressions or written by the user). Then a 
request needs to also include not-yet-committed features. These need to 
be evaluated as well. In case the user directly specifies sql for the 
provider and not a QgsExpression this will pose some more problems.

A new caching mechanism has been merged recently, but I don't expect 
any performance improvements yet. The new caching mechanism 
nevertheless supports plugable indexes, which I hope can help at least 
in part of these situations as soon as they are implemented.

The only possibility to make use of the data providers indexes that I 
know of right now (apart from making views et al) is to provide a 
subset string in the layer properties, but that's most likely not what 
you want.

Also, as mentioned by Denis Rouzaud, the rendering that takes when the 
selection changes place takes its bit of the performance. I remember, 
that somebody began a list of such unnecessarily triggered renders, but 
can't find it right now. I hope that in this area some work will be 
done as well.

Kind regards,
Matthias

On Mit 27 Mär 2013 20:57:07 CET, Régis Haubourg wrote:
> Hi dev's, I was notified some performance matters when dealing with
> selections in attribute tables, by some users swithcing from arcgis to QGIS.
> Indexes of datasource seem to be ignored for SQL selection and search. This
> leads to very slow selections when dealing with large datasets.
> Anyone confirms?  Is it really related to indexes or another bottleneck?
> Is there any possible improvement ? Are providers working on selections or
> is that all dealing with cache?
>
> Thanks for your feedback ,
> Régis
>
>
>
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