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The dataprovider from a vector layer returned from vectorLyr.dataProvider(). Also the same thing happens against just the vectorLyr, so vectorLyr.minimumValue(index) returns the same 0.0 for a null value. The layer is not in edit mode and does not contain changes.

And which data provider as in postgis/spatialite/ogr ...?

ogr -- shapefile

Are the OGR versions on your Windows and Linux machine the same?

windows ogr 1.11.1

linux ogr 1.9.2

Yes, just created a ticket over there. Thank you for all of the help.

By looking at this ESRI document http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Geoprocessing_considerations_for_shapefile_output NULL values in a numeric field are really stored as zeroes into the .dbf part of shapefile. And if NULL is stored as a zero into the .dbf file there is no way for GDAL to separate it from zero that represents a real zero. I may be wrong but this ticket and the corresponding GDAL ticket feel invalid for me. But I do not understand why Linux makes a difference for you. Could you add a shapefile with a few features and some NULLs for testing?

GDAL ticket 5333 is not related, it was about creating new functions for finding min/max/avg dates from a DATE field of a shapefile which actually a string "CCYYMMDD" in the .dbf file.

I can reproduce the problem on Linux with GDAL 1.11

Taking the shapefile lakes from here:

https://github.com/opengisch/QGIS-Sampledata/tree/master/shapefiles

Minimum value for the xlabel column (values 28399 and a some NULLs):

iface.activeLayer().dataProvider().minimumValue( 3 )

0

Editing the attriubte table and changing a NULL value to 0 works. The change can be saved and reloaded properly, so GDAL seems to be able to distinguish 0 and NULL for shapefiles.

Upstream issue: https://trac.osgeo.org/gdal/ticket/5799

I downloaded the lakes shapefile and had a try with GDAL 2.0-dev on Windows 7 but this version does not show the issue with ogrinfo:

ogrinfo lakes.shp -sql "select min(xlabel) from lakes"
INFO: Open of `lakes.shp'
using driver `ESRI Shapefile' successful.

Layer name: lakes
Geometry: None
Feature Count: 1
Layer SRS WKT:
(unknown)
MIN_xlabel: Integer (8.0)
OGRFeature(lakes):0
MIN_xlabel (Integer) = 28399

Hmmm, same result here with 1.11.1.

Debugging shows that ogr fails to interpret the sql qgis sends it (see below) and it's actually our fallback code that fails to properly handle NULL values.

QGIS debug output:

src/providers/ogr/qgsogrprovider.cpp: 2360: (minimumValue) [30361ms] Failed to execute SQL: SELECT MIN("xlabel") FROM "lakes" 

ogrinfo output:

ogrinfo lakes.shp -sql 'SELECT MIN("xlabel") FROM "lakes"'
INFO: Open of `lakes.shp'
      using driver `ESRI Shapefile' successful.
ERROR 1: Argument of column Summary Function 'MIN' should be a column.

Therefore the proper question is, should we send an unquoted column name to ogr or should that be changed on gdal side?