[Qgis-developer] Processing algorithms slower when using filters on vector layers

[G. Allegri]

I've noticed that when a (OGR) layer has a filter expression applied on it,
processing algorithms perform worse.
I have tested several polygon layers with some thousands of features. If I
do, e.g., an intersection, it takes a lot more if I apply a filter that
reduces the number of features to some dozens, then without filtering.

I guess it can be caused by the extra work that the feature irerator has to
do with the filter (request with expression) but it's an unexpected
behaviour, at least for many of my courses partecipants.

Tested on QGIS 2.14.10 and 2.18.4.

giovanni
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