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I'm using QGIS 3.2.1 (Bonn)

I'm trying to dissolve and create statistics from a group of polygons, but QGIS is not making it. I do not know if I'm missing something or the software has a bug. These are my steps:

1- Select a group of polygons from a layer

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2- Use the "Dissolve" tool of the GDAL add-on.

3- Characteristics to dissolve:

Input layer: "Census ratios", check.

Selected features only, check.

Dissolve field, "Dissolve_" (I have created a column with the same attribute "AAA" for all rows for this purpose), check.

Compute stats for attribute, check. Attribute selected: Total population (it is an interger64 field), check.

Now take a look at the console: Dissolve_, SUM(Dissolve_) AS sum, MIN(Dissolve_) AS min, MAX(Dissolve_) AS max, AVG(Dissolve_) AS avg FROM layer_name GROUP BY Dissolve_" -f "ESRI Shapefile"

Is using the Dissolve_ field to calculate the stats. Why? I don't know.

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Then go, and the results are crearly wrong. I want to summarize the TOTAL_POPULATION field, not the one used to dissolve the polygons (DISSOLVE_).

enter image description here

Is QGIS failing because of a bug? Or i'm doing something wrong?

Note: I've tried with ArcMap and it goes OK, but i need to work with QGIS.

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  • Yep, I'm running into this too...
    – DWal
    Sep 20, 2018 at 1:56

2 Answers 2

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Confirmed, it's an issue with the GDAL plugin. Fix ready to be implemented on 3.4.0 release.

Temporary solution:

Andrea Giudiceandrea wrote: PR https://github.com/qgis/QGIS/pull/7964 submitted for QGIS 3.3.0-master

You can fix your Dissolve algorithm python script by changin line 157 in [QGIS install dir]\apps\qgis\python\plugins\processing\algs\gdal\Dissolve.py

from

statsField = self.parameterAsString(parameters, self.FIELD, context)

to

statsField = self.parameterAsString(parameters, self.STATISTICS_ATTRIBUTE, context)

source: https://issues.qgis.org/issues/19900

Regards!

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Could you try using a different Field type rather than Integer64? I would use Integer for a field containing a population number or a decimal if they are not whole numbers.

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  • Already did it, same result. The console continues using the same incorrect field to calculate the stats ("Dissolve_") : ogr2ogr C:/Users/Facu/AppData/Local/Temp/processing_f783d348c9ed4e5d8b5c31613c1bde55/c2a0ca4a8b35436e8781aa0efec75d9f/OUTPUT.shp path_to_data_file -dialect sqlite -sql "SELECT ST_Union(geometry) AS geometry, Dissolve_, SUM(Dissolve_) AS sum, MIN(Dissolve_) AS min, MAX(Dissolve_) AS max, AVG(Dissolve_) AS avg FROM layer_name GROUP BY Dissolve_" -f "ESRI Shapefile".
    – facu pedemonte
    Sep 20, 2018 at 0:51

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