[Qgis-developer] Multiple geometry columns per layer

Matthias Kuhn matthias.kuhn at gmx.ch
Thu Nov 29 06:15:45 PST 2012

On 11/29/2012 03:06 PM, Nathan Woodrow wrote:
> Hey,
> Honestly I find that a pain in the neck, from a users point of view 
> and from a developer point of view.  From a users point of view you 
> now have to have a tools that supports styling points, regions, lines 
> all at the same time, it also makes processing harder because you have 
> to check the geometry type before doing certain operations.
> From a developer point of view I think it can make things hard because 
> you have to check feature by feature what kind geometry you are 
> dealing with before working on it, have you got a 0 length line or a 
> point because length of a point is 0?. At the moment we just open the 
> layer, check the type and select the correct methods.
Aren't you mixing up multiple geometry columns and type safety? I'm not 
talking about different geometry types in one column, but in different 
columns, so it should always be clear, from a certain combination 
layer/column, which tools/actions are available.
> MapInfo does this and I always found it caused more problems then it's 
> worth IMO.  Although postgres and spatialite support storing different 
> geometry types in the same table I have seen it as bad practice.  If 
> you do happen to have a layer with say regions and points for building 
> footprints, you can put the layer twice in the geometry_columns table 
> with a different geometry type and QGIS will pick it up as a different 
> layer, then you can just set the zoom levels on each layer.
My problem now is, that I need to use different geometries for the same 
feature from a plugin. As the database is sometimes really slow, I'd 
prefer to fetch just one table with the geometries in several columns. 
This way, I also would not have to join the two or three tables again in 
a plugin (it's error prone and slower the database could handle this)
> They are my thoughts anyway.
> - Nathan
> On Thu, Nov 29, 2012 at 11:55 PM, Matthias Kuhn <matthias.kuhn at gmx.ch 
> <mailto:matthias.kuhn at gmx.ch>> wrote:
>     Hi,
>     Why is not more than one geometry column per layer supported?
>     Would it be hard to support more than one geometry column (and set
>     one as the default to be displayed)?
>     It would certainly make some tasks easier and maybe could be used
>     for level of detail, related to the current zoom level.
>     There was recently a discussion on this list about spatialite as
>     default layer type and spatialite would support this, as does
>     postgres.
>     Regards,
>     Matthias
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