[processing] Optimise uniqueValues method

Now it uses the standard QgsVectorLayer.uniqueValues() method
where possible so that provider side optimisations are used

Also add test, and optimise request when using selected
features only

python/plugins/processing/tests/ToolsTest.py

@@ -122,6 +122,32 @@ def testValues(self):
 
         ProcessingConfig.setSettingValue(ProcessingConfig.USE_SELECTED, previous_value)
 
+    def testUniqueValues(self):
+        ProcessingConfig.initialize()
+
+        test_data = points2()
+        test_layer = QgsVectorLayer(test_data, 'test', 'ogr')
+
+        # field by index
+        v = vector.uniqueValues(test_layer, 3)
+        self.assertEqual(len(v), len(set(v)))
+        self.assertEqual(set(v), set([2, 1, 0]))
+
+        # field by name
+        v = vector.uniqueValues(test_layer, 'id_2')
+        self.assertEqual(len(v), len(set(v)))
+        self.assertEqual(set(v), set([2, 1, 0]))
+
+        # test with selected features
+        previous_value = ProcessingConfig.getSetting(ProcessingConfig.USE_SELECTED)
+        ProcessingConfig.setSettingValue(ProcessingConfig.USE_SELECTED, True)
+        test_layer.selectByIds([2, 4, 6])
+        v = vector.uniqueValues(test_layer, 'id')
+        self.assertEqual(len(v), len(set(v)))
+        self.assertEqual(set(v), set([5, 7, 3]))
+
+        ProcessingConfig.setSettingValue(ProcessingConfig.USE_SELECTED, previous_value)
+
 
 if __name__ == '__main__':
     unittest.main()

python/plugins/processing/tools/vector.py

@@ -122,13 +122,23 @@ def uniqueValues(layer, attribute):
     Attribute can be defined using a field names or a zero-based
     field index. It considers the existing selection.
     """
-    values = []
+
     fieldIndex = resolveFieldIndex(layer, attribute)
-    feats = features(layer)
-    for feat in feats:
-        if feat.attributes()[fieldIndex] not in values:
-            values.append(feat.attributes()[fieldIndex])
-    return values
+    if ProcessingConfig.getSetting(ProcessingConfig.USE_SELECTED) \
+            and layer.selectedFeatureCount() > 0:
+
+        # iterate through selected features
+        values = []
+        request = QgsFeatureRequest().setSubsetOfAttributes([fieldIndex]).setFlags(QgsFeatureRequest.NoGeometry)
+        feats = features(layer, request)
+        for feat in feats:
+            if feat.attributes()[fieldIndex] not in values:
+                values.append(feat.attributes()[fieldIndex])
+        return values
+    else:
+        # no selection, or not considering selecting
+        # so we can take advantage of provider side unique value optimisations
+        return layer.uniqueValues(fieldIndex)
 
 
 def resolveFieldIndex(layer, attr):