[Qgis-developer] Way to improve attribute search

[Martin Dobias]

On Tue, Feb 9, 2010 at 9:09 AM,  <luca_manganelli at comune.trento.it> wrote:
>
> Hi,
>
> in my python plugins I always use a simple procedure to search features by
> value. I get every single feature in a loop, get the n-th field value, I
> compare it with a number or a substring.
> The problem is that the performance is poor (with shapefiles / postgis
> layers with over 10.000 features).
> But I don't know if there is another way to improve the performance. And,
> sadly, it doesn't use the power of Postgres query to search features with
> given in a boost of time.

Can you give a code sample of such search?
What is the average time it takes and what time do you need?

I believe any simpler searches could be implemented in c++ e.g. by
creating a new static method:
QList<QgsFeature>
QgsSearchString::findMatchingFeatures(QgsVectorLayer* layer, QString
searchString);

So, given a vector layer and a condition ("ROAD_TYPE = 3 OR ROAD_TYPE
= 5") it will return a list of features that match. Since the whole
search would be done within c++ code, that would avoid performance
penalty of python code with lots of iterations.

In case you need really fast searching, you could create an index -
especially if the queries are for the same field (or same set of
fields). Another choice would be to cache the whole features if there
aren't millions of them.

Martin