[Qgis-developer] virtual fields

Andreas Neumann a.neumann at carto.net
Mon Mar 3 07:16:11 PST 2014

Hi Denis,

I proposed this a couple of years ago:

But no-one dared to implement it until now.

Meanwhile the expressions are almost like "virtual columns" - but for
some purposes it would still be good to have (e.g. in forms, for QGIS
server outputting calculations, etc.). I can also see virtual columns
that are aggregates of linked tables (e.g. the sum/min/max of linked

So I would also like to support such a development if someone wants to
do the work.


Am 03.03.2014 14:49, schrieb Denis Rouzaud:
> 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.
> 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.
> I believe this is a deep refactoring of the fields behavior, isn't it?
> This could be a chance to clean up the joins, which I believe are a bit
> messy now.
> This is totally above my skills and somehow my available time, but I
> would have (a part of) funding for this.
> What do you think of this idea?
> Would it be "easily"achievable without breaking the API?
> Would you be interested in taking part of the dev?
> Cheers,
> Denis
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