[Qgis-user] Selection issue for large set of features

[Bernhard Ströbl]

Hi Yves,

why does the featureId not point to the PK? Can you guarantee that the 
value is unique in the view? Maybe it is not?
Another thought: I normally use QgsVectorLayer.setSubsetString() to 
reduce the amount of features being loaded into QGIS in the first place 
(and by using the subsetString I let PG deal with it). Thus I can use 
plain SQL and do not have to fuzz with QgsFeatureRequest.

hope this helps

Bernhard

Am 09.12.2015 um 18:36 schrieb Yves Moisan:
> Hi All,
>
> Sorry for X-posting.  I originally put this message on the qgis-dev list
> but I guess it wasn't the right place.
>
> I'm contributing to a QGIS python plugin.  Dev environment : 2.8.3
> (compiled) Ubuntu 14.04.
>
> I have an issue getting a list of PK ids from selected features on a
> vector layer pointing to a PG view .  The short story is : I would like
> to use selectedFeaturesIds() but I can't rely on ids returned being the
> actual PK.  The longer story now.
>
> I can use QGSFeature objects and get the field I want.  However, all
> variants that give a list of QGSFeature objects that I've tried fail
> given a large enough number of selected features :
>
> # getting all features with attributes and geom; definitely overkill
>
> layer_selected_features = layer.selectedFeatures()  # Then extract the
> attribute I want
>
> # getting all features but only with the (predetermined) table_pk;
> layer_selected_features_ids was obtained by selectedFeaturesIds()
>
> layer.getFeatures(QgsFeatureRequest().setFilterFids(layer_selected_features_ids).setFlags(
> QgsFeatureRequest.NoGeometry ).setSubsetOfAttributes(
> [table_pk],layer.pendingFields() )))  # Then extract the attribute I want
>
> A note on the previous line : I found out order of the filtering
> elements matters.
>
> # Using a feature iterator
>
> layerSelectedFeaturesIterator = layer.selectedFeaturesIterator()
> layer_selected_features_ids = list(i[table_pk] for i in
> layerSelectedFeaturesIterator) # I get directly the list of items I want
>
> When the feature selection gets large enough the code fails silently and
> no list is returned.
>
> I was hoping I could use selectedFeaturesIds().  I tried it with my
> largest data sets and it works.  Only problem is : the id list it
> returns may or may not correspond with the actual table PK the view
> points to.  setKeyColumn() on the layer won't make selectedFeaturesIds()
> return a list of ids with the new keyColumn().  I was thinking maybe I
> could pass in the field name for the PK (that I know) to some
> constructor (datasource, layer ?) and then be satisfied the list of ids
> returned by selectedFeaturesIds() would be fine.
>
> Pointers appreciated.
>
> Cheers,
>
> Yves
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user
> Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user



__________ Information from ESET Mail Security, version of virus signature database 12699 (20151210) __________

The message was checked by ESET Mail Security.
http://www.eset.com