Make sure that relevant fields are set always when necessary.

src/providers/ogr/qgsogrprovider.cpp

@@ -556,6 +556,10 @@ void QgsOgrProvider::setRelevantFields( bool fetchGeometry, const QgsAttributeLi
 
     OGR_L_SetIgnoredFields( ogrLayer, ignoredFields.data() );
   }
+
+  // mark that relevant fields may not be set appropriately for nextFeature() calls
+  mRelevantFieldsForNextFeature = false;
+
 #else
   Q_UNUSED( fetchGeometry );
   Q_UNUSED( fetchAttributes );
@@ -623,6 +627,14 @@ bool QgsOgrProvider::nextFeature( QgsFeature& feature )
     return false;
   }
 
+  if ( !mRelevantFieldsForNextFeature )
+  {
+    // setting relevant fields has some overhead so set it only when necessary
+    setRelevantFields( mFetchGeom || mUseIntersect || !mFetchRect.isEmpty(),
+                       mAttributesToFetch );
+    mRelevantFieldsForNextFeature = true;
+  }
+
   OGRFeatureH fet;
   QgsRectangle selectionRect;
 
@@ -726,6 +738,7 @@ void QgsOgrProvider::select( QgsAttributeList fetchAttributes, QgsRectangle rect
 
   setRelevantFields( mFetchGeom || mUseIntersect || !mFetchRect.isEmpty(),
                      mAttributesToFetch );
+  mRelevantFieldsForNextFeature = true;
 
   // spatial query to select features
   if ( rect.isEmpty() )

src/providers/ogr/qgsogrprovider.h

@@ -334,6 +334,13 @@ class QgsOgrProvider : public QgsVectorDataProvider
     int geomType;
     long featuresCounted;
 
+    /** Flag whether OGR will return fields required by nextFeature() calls.
+        The relevant fields are first set in select(), however the setting may be
+        interferred by some other calls. This flag ensures they are set again
+        to correct values.
+     */
+    bool mRelevantFieldsForNextFeature;
+
     //! Selection rectangle
     OGRGeometryH mSelectionRectangle;
     /**Adds one feature*/