[Qgis-developer] Thoughts about multi-type tables in QGIS

[Denis Rouzaud]

Hi Olivier,

I think you can easily extend your reasoning to the support of multiple 
geometry columns in QGIS.
I believe that this would come in QGIS at some point.

If QGIS 3 is getting closer, our company will support this feature.

As you suggest, the layer could have different geometry types and/or 
columns. This could be seen as sub-layer level for rendering:
layer > geometry types/columns > symbology rules

I suppose this is quite a big change. Many things depends on the 
geometry type in QGIS (like map map tools), and that would represent a 
large API break.

But, on my side, I believe it's a logic evolution for QGIS.

Best wishes,

Denis





On 04/02/2015 01:52 PM, Olivier Dalang wrote:
> Hi,
>
> In some projects of mine, I work with multiple geometry types in one 
> postgis table, using a column of type geometry(Geometry,4326).
> This is very well supported by postgis.
>
> It is possible to load such a table in QGIS by manually selecting the 
> geometry type you want to load. This means that to display all the 
> features, you need to add the table three times, one for each feature 
> type.
>
> This works more or less. There are a few bugs though :
> - http://hub.qgis.org/issues/12499 (you can edit other type's node 
> with the node tool)
> - http://hub.qgis.org/issues/12500 (other type's records are shown in 
> the attribute table)
>
> This also has some limitations. When having such a setup, it's pretty 
> sure you'll want to have the same edit forms for all the layers. 
> You'll also probably want the same filter, the same labels, the same 
> actions, etc...
> The only thing you'd want to be able to define on a geometry type 
> basis are the symbol (well, even the classification/colors/etc could 
> be shared) and the label placement.
> For now, you must do all settings three times, because of this 
> bug/feature request :
> - http://hub.qgis.org/issues/12303 (copy/paste style from one geometry 
> type to another)
>
>
> As you see, support multiple geometry types in QGIS is not perfect.
>
> Of course it's possible to fix the bugs/pr, and there are some 
> workarounds (postgis view instead of tables) but maybe it's also worth 
> thinking a bit more in depth about this.
>
> We could consider point/line/polygons as subcategories/sublayers of a 
> layer. A shapefile or a mono-typed table would have only one of those 
> sublayer, but a postgis table could perfectly have the three. Most of 
> the settings would be defined at the layer level, while only some 
> settings would be defined at the subcategory level.
>
> This is probably especially relevant when thinking long term (the day 
> we support 3D, curves, etc...).
>
>
> What do you think ?
> Do you think the relation 1 layer = 1 geometry type will hold ?
>
> I think we inherited this from the old shapefile format, but most data 
> sources QGIS handles don't have this limitation. I also think it does 
> not hold with quite a lot of modern GIS uses (especially web related, 
> think of openstreetmaps for instance).
>
> There's this feature request (6th oldest open issue on the tracker) 
> about postgis geometry collections  : http://hub.qgis.org/issues/167
>
>
> Best,
>
> Olivier
>
>
>
>
>
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