[OGR provider] Support full SELECT subset string

This will be useful for DBManager SQL request layer.

Author: rouault

src/providers/ogr/qgsogrprovider.cpp

@@ -3251,8 +3251,14 @@ OGRLayerH QgsOgrProviderUtils::setSubsetString( OGRLayerH layer, OGRDataSourceH
       layerName = encoding->fromUnicode( modifiedLayerName );
     }
   }
-  QByteArray sql = "SELECT * FROM " + quotedIdentifier( layerName, ogrDriverName );
-  sql += " WHERE " + encoding->fromUnicode( subsetString );
+  QByteArray sql;
+  if ( subsetString.startsWith( "SELECT ", Qt::CaseInsensitive ) )
+    sql = encoding->fromUnicode( subsetString );
+  else
+  {
+    sql = "SELECT * FROM " + quotedIdentifier( layerName, ogrDriverName );
+    sql += " WHERE " + encoding->fromUnicode( subsetString );
+  }
 
   QgsDebugMsg( QString( "SQL: %1" ).arg( encoding->toUnicode( sql ) ) );
   return OGR_DS_ExecuteSQL( ds, sql.constData(), nullptr, nullptr );

tests/src/python/test_provider_ogr_gpkg.py

@@ -200,5 +200,27 @@ def testGeopackageExtentUpdate(self):
         self.assertTrue(QgsGeometry.compare(provider_extent.asPolygon()[0], reference.asPolygon()[0], 0.00001),
                         provider_extent.asPolygon()[0])
 
+    def testSelectSubsetString(self):
+
+        tmpfile = os.path.join(self.basetestpath, 'testSelectSubsetString.gpkg')
+        ds = ogr.GetDriverByName('GPKG').CreateDataSource(tmpfile)
+        lyr = ds.CreateLayer('test', geom_type=ogr.wkbMultiPolygon)
+        lyr.CreateField(ogr.FieldDefn('foo', ogr.OFTString))
+        f = ogr.Feature(lyr.GetLayerDefn())
+        f['foo'] = 'bar'
+        lyr.CreateFeature(f)
+        f = None
+        f = ogr.Feature(lyr.GetLayerDefn())
+        f['foo'] = 'baz'
+        lyr.CreateFeature(f)
+        f = None
+        ds = None
+
+        vl = QgsVectorLayer('{}|layerid=0'.format(tmpfile), 'test', 'ogr')
+        vl.setSubsetString("SELECT fid, foo FROM test WHERE foo = 'baz'")
+        got = [feat for feat in vl.getFeatures()]
+        self.assertEqual(len(got), 1)
+
+
 if __name__ == '__main__':
     unittest.main()