Select distinct does not return NULL values when they are in the first records

[ndawson]

Running a "SELECT DISTINCT..." query from a layer containing NULL values does not include a NULL value in the returned results.

Here's the results of running this query on the shapefile available at ​https://github.com/qgis/QGIS/tree/master/tests/testdata/provider

ogrinfo -sql "SELECT DISTINCT name FROM shapefile" shapefile.shp

INFO: Open of `shapefile.shp'

using driver `ESRI Shapefile' successful.

Layer name: shapefile Geometry: Point Feature Count: 4 Layer SRS WKT: (unknown) name: String (80.0) OGRFeature(shapefile):0

name (String) = Pear

OGRFeature(shapefile):1

name (String) = Orange

OGRFeature(shapefile):2

name (String) = Apple

OGRFeature(shapefile):3

name (String) = Honey

Removing the "DISTINCT" clause shows that null values are present:

ogrinfo -sql "SELECT name FROM shapefile" shapefile.shp

INFO: Open of `shapefile.shp'

using driver `ESRI Shapefile' successful.

Layer name: shapefile Geometry: Point Feature Count: 5 Extent: (-71.123000, 66.330000) - (-65.320000, 78.300000) Layer SRS WKT: GEOGCS["GCS_WGS_1984",

DATUM["WGS_1984",

SPHEROID["WGS_84",6378137,298.257223563]],

PRIMEM["Greenwich",0], UNIT["Degree",0.017453292519943295]]

name: String (80.0) OGRFeature(shapefile):0

name (String) = (null) POINT (-71.123 78.23)

OGRFeature(shapefile):1

name (String) = Pear

OGRFeature(shapefile):2

name (String) = Orange POINT (-70.332 66.33)

OGRFeature(shapefile):3

name (String) = Apple POINT (-68.2 70.8)

OGRFeature(shapefile):4

name (String) = Honey POINT (-65.32 78.3)

[Even Rouault]

The issue was specific only if the records with NULL values were before records with non-NULL values.

trunk r29481, branches/2.0 r29482 "OGR SQL: do not silently skip NULL values in the first records when evaluating a SELECT DISTINCT (#6020)"