[Qgis-developer] Questions/Suggestion on virtual layers

Neumann, Andreas a.neumann at carto.net
Wed Mar 2 08:48:06 PST 2016

Hi Hugo, 

On 2016-03-02 17:17, Hugo Mercier wrote:

> Hi Andreas,
> On 02/03/2016 16:27, Neumann, Andreas wrote: 
>> Hi,
>> I am experimenting with the virtual layers. They are quite cool!
>> I have some suggesions/questions:
>> 1. It would be nice to have a preview of my query results. Currently it
>> is either it works - or not. If it works, but the results are wrong
>> you have to start from scratch. I would suggest that when you press
>> on the test button you would not only display "No error" or the
>> error message, but maybe the first 10 rows of the result.
> True.
> Note that you can also create a virtual layer using the DB Manager where
> it can display the first rows of a query, as it is the case for other
> databases.
> I am not sure to love having these two very close but slightly different
> dialogs. If anyone has a better idea in terms of usability, do not
> hesitate to propose :)

Can one create new virtual layers in the DB manager? I know that I can
do queries on existing layers and load it as a new layer - but is this
the same thing like a virtual layer? 

>> 2. Updating the virtual layer: it would be really nice if I could
>> edit/improve on my virtual layers without having to start completely
>> from scratch. --> Oh - now I found out that if you select an
>> existing virtual layer and then use "Layer" --> "Add Layer" --> "Add
>> Virtual Layer", that you can refresh an existing virtual layer. This
>> is really not obvious. I think this is more a usability issue.
> Yes ... I am more used to click on the addition icon, which makes it
> more direct, but you are right, this is not perfect.

ah ok - the button works as "edit" if you have the layer selected. Maybe
then we should rename the tooltip from "Add Virtual Layer" to "Add or
Edit Virtual Layer" 

> An entry in the right-click menu ?

Yes - that was my first idea - to look for an entry in the layer context
menu. Of course this entry should only be there if it is a virtual

> Edit - you can also use the drop down menu of the creation dialog to
> switch between different virtual layer definitions, as mentioned by Yves

yes - now I found it - thanks! 

> 3. it is not very obvious how you can reuse an already existing
> geometry column. I discovered it by accident, that you can use
> layer.geometry (regardless of the original name of the geometry
> column), but I think that it is not obvious. A better hint in the
> the tooltip would probably help.
> Indeed it appears to be not documented.

This would just require an additional sentence in  the tooltip of the
Query panel, after the sentence "Any functions from SQLite or Spatialite
can then be used in the query.: 

To add or access existing geometries of a table, you can use
"tablename.geometry", regardless of original geometry column's name. 

> 4. Unique identifier column: If you join several tables, you often have
> the issue that you need to create an artificial primary key. Could
> we offer a preconfigured builtin pkey - something like a rowid  for
> such cases?
> Good point. An autoincremented id is already generated on the feature.
> That may not be a problem to add a "rowid" column in that case.

That would be cool to have. 

> 5. What is the exact purpose of the "Embedded Layers" where you can
> add/import/remove layers? I noticed that existing layers of the
> project work anyway without adding/importing them first. Is the idea
> that you can work on layers that aren't in the project?
> Yes it is.
> You can for example embed layers from the project, save the virtual
> layer to a QLR and reload it in a new project, the referenced layers
> would not need to be already loaded.
> There may be some documentation needed as well here.

Maybe we could add another tooltip here, explaining that one can add
layers that are not in the project - and that you need to "embed
existing layers", if you want to save to a qlr and load it in a
different project. 

> Should I open feature request tickets for these or are there already
> plans to address one or the other of the above issues?
> Unfortunately, no plan yet.
> Thanks for your tests and feedback !

Ok - I will open feature requests for the above, where useful. 

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