Hi I have many people asking for this kind of "virtual fields". The first time I show people the field calculator during curses, people often tell me :
* I have done a mistake, how can I change the formula and see the result updated ? -> they often compare this feature to the excel formula * I do not want to create a new column, just use it for labelling/styling . I answer them "Just use expressions in the label/style pannel" and they answer back "But how can I preview the results in the attribute table for all features before ?" This is definitely a nice feature to have. We must take performance into account though, for big datasets. cheers Michael 2014-03-03 16:36 GMT+01:00 Martin Dobias <[email protected]>: > Hi Denis > > On Mon, Mar 3, 2014 at 9:49 PM, Denis Rouzaud <[email protected]> > wrote: > > Hi all, > > > > I was wondering about the idea of being able to create simple views in > QGIS, > > "views" in terms of database. > > Often, I use views (in postgres) to be able to create labels, title, > simple > > calculation or whatever and also to combine data from joined tables. > > > > Somehow, all is already there in QGIS but in distinct features: > > - joins which will add the fields of the joined table to the main table > > - expressions are used in many places where views are meaningful: labels, > > styling, search, etc. > > - field calculation > > > > I was thinking that these features should be grouped and improved in what > > could be called virtual fields -- so basically a "live" field calculation > > without writing to the provider. > > > > First, the idea would be to define virtual fields using expressions. > > I have thought about this functionality before and I think it should > be relatively easy to add this functionality. The > QgsVectorLayerFeatureIterator could simply compute the values of the > virtual fields according to their definition in QgsVectorLayer. It > should automatically work lazily - if the field is not requested, it > would not be evaluated. > > > > Then, it should be made possible to integrate the joined fields in qgis > > expressions without adding them to the current attributes (as it is now). > > One way to to it could be a function: joinField( joinName, > joinedFieldName) > > Hence, the user would be allowed to choose which joined fields he really > > wants to be accessible in the layer. > > Do we actually need such functionality? Wouldn't it be enough to hide > the attributes you don't want? > > > Regards > Martin > _______________________________________________ > Qgis-developer mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/qgis-developer >
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