[QGIS-Developer] Questions wrt. problems with empty layers

Thu May 9 13:22:03 PDT 2019

Hi Lars and other members of this list..

I feel somewhat obligated to help Lars since it's a problem he has 
inherited from me. (Sorry about that...)

To sum it up:

If QGIS connects to a WFS based layer from Geoserver using an existing 
project with styled layers and one or more layers containing /zero/ 
objects, QGIS can't determine the object type (point, line-string, 
polygon...) for those layers. QGIS will not show legends  and promptly 
forget any layer information stored in the project related to the 
object-type, ex. styling information.

And what is worse - this is a permanent situation: If you save the 
project in the above-mentioned state and reopen it at a later time - 
where the layers /now/ contains objects - QGIS has forgotten the styling 
and other information, so the layers stays invisible and without the 
former styling.

(long posting...)

I've done some tests to determine the root cause of this problem:

  * Setup of a Postgres database /and /a SQL Server database.

  * Loaded the same MapInfo-dataset twice into each database, called
    them tableA and tableB, and populated the meta-table
    "dbo.geometry_columns" with the correct information for tableA and
    tableB in SQLServer.

  * Installed a fresh Geoserver 2.15 and MapServer TinyOWS WFS-T server

  * Setup a Workspace in Geoserver with 4 layers: tableA and -B from
    SQLServer and the same tables from Postgres using WFS.

  * In Geoserver, the datastore definition for the SQLServer tables has
    a pointer to the "dbo.geometry_columns" meta-table.

  * Setup of TinyOWS with tableA and -B from Postgres

  * And lastly: Made a project in QGIS 3.4.7 /Windows 64bit with the 6
    WFS layers:

      o tableA is shown 3 times:

         1.  From SQLServer through GeoServer;
         2.  From Postgres through GeoServer
         3.  From Postgres through Tiny-WFS

      o tableB is shown 3 times using the same setup as tableA

All 6 layers in QGIS was shown correctly and the object type for all 
layers was recognized.

After this I /deleted/ all rows from tableB in /both/ MS-SQLServer and 
in Postgres and reopened the same QGIS project:

  * The layer showing (the zero objects) tableB from SQLServer through
    GeoServer was not in order.
  * The 5 other layers was shown correctly. Legends was correct and
    styling was not deleted

I'm drawing the following conclusion:

  * GeoServer WFS connections (maybe in conjunction with QGIS) doesn't
    work properly when the dataprovider is MS-SQLServer and the layers
    contains zero objects. More precisely: GeoServer's SQLServer
    dataprovider can't or don't use geometry_columns meta-table defined
    in the datastore as described in:
    https://docs.geoserver.org/stable/en/user/data/database/sqlserver.html#using-the-geometry-metadata-table
    .

After this test I've tried to change the placement and existence of the 
"geometry_columns" metadata table:

  * No value
  * dbo.geometry_column (ordinary placement)
  * data.geometry_column (I made a copy of the geometry_columns table
    and placed it in the same schema as the test tables)
  * geometry_columns (without schema definition)

Nothing of the above worked.

@Lars:
You might change your database system to Postgres and perhaps replace 
GeoServer with TinyOWS for WFS purposes (ducks and and run....) .
Or getting the issue investigated and fixed using some GeoServer core 
developer, perhaps in cooperation with a QGIS core developer?
Apparently the problem is fixable, since neither TinyOWS or 
GeoServer/Postgres have the same  issues.

Obviously, the above is a far cry from a really rigorous investigation 
of the problem, but it points at some of the "pain" points

Regards

Bo Victor Thomsen

AestasGIS
Denmark

Den 09-05-2019 kl. 13:36 skrev Lars I. Nielsen, LIFA A/S:
>
> Hi developers.
>
> I asked this question on the user’s list, so far without any answers, 
> but have been advised to ask here as well.
>
> I’m working with a number of projects, which statically open a fixed 
> number of layers. All layers are fetched using WFS / WFS-T.
>
> Each layer has an extensive rule based styling, and an extensive 
> field/attribute dialog setup. This is saved in the project files.
>
> Our plugin changes the data provider filter for many layers, when the 
> user works with the data. These filter expressions are also saved in 
> the project, of course.
>
> Our problem is, that if the user accidently chooses a work area (= new 
> data provider filter), which contain no data in any layer, those 
> layers lose all styling etc. And it’s apparently impossible to recover 
> these styles etc. by changing the filter back.
>
> So basically the user’s project is unrecoverably trashed at this point 
> in time.
>
> A - Is this a correct description of how QGIS handles “no-data” layers ?
>
> B - Are there any standard ways to handle this in QGIS (using metadata 
> or otherwise) in an automated fashion (i.e. not manually) ?
>
> C - If the answers to A and B are yes and no resp., are there any 
> plans to remedy this unnecessarily serious problem ?
>
> Might it not be solved simply by adding an (optional?) layer metadata 
> key, that stores the last used topology, and revert to this if no data 
> are found on the layer ?
>
> Using 3.4.5 x64 (Danish)
>
> Med venlig hilsen
>
> Lars I. Nielsen, LIFA A/S
> GIS-konsulent, FME Certified Professional
>
> Beskrivelse: http://website.lifa.dk/lsp.gif
>
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>
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