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

[Olivier Dalang]

Hi,

Would it really be more complicated ?

I mean, for now, an algorithm that works only with line layers already has
to check whether the layer is of type line. That's done before iterating
the features.

Exactly in the same way, there would be functions to determine whether a
layer supports a geometry type or not.

Then, there would be functions to iterate a particular geometry type for a
layer.
This could be done by adding a geometry type argument to getFeatures,
and/or by adding specific
getLineFeatures/getPointFeatures/getPolygonFeatures functions, probably
throwing exceptions if the layer does not support this particular type
(shapefile or specific geometry column in postgis).

To me it seems not much different from how it works currently, at least
from a programmer's point of view. Of course it's quite a change in the
API, that's why it's only thoughts for a future QGIS 3 or 4...


I don't know Mapinfo at all, but it's good to know there's already some
experience somewhere (even if bad). But do you really think the problem is
in the principle itself, and not in Mapinfo's implementation ?


The things at stake are maybe worth the thought still...
The heavy distinction between geometry types is very artificial. There's a
lot of very valid representation of real world phenomenons of a certain
kind that require different geometry types. Having to distribute those
across different layers because of a 25 years old file format is somewhat
sad...


Olivier


2015-04-02 16:41 GMT+02:00 Bo Victor Thomsen <bo.victor.thomsen at gmail.com>:

>  As an old MapInfo user/developer my opion is: Don't do it. It has always
> been a problem in MapInfo and it will be a problem in QGIS - if
> implemented.
>
> A better approach is to have the possibility to let different QGIS layers
> share some common characteristics (for example labelling). And - of course
> - clean up the current errors in QGIS when splitting contents of data
> sources by object types.
>
> Regards
> Bo Victor Thomsen
> AestasGIS
> Denmark
>
> Den 02-04-2015 kl. 13:52 skrev Olivier Dalang:
>
>  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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