Use expression filter request for SelectByAttribute

Makes SelectByAttribute orders of magnitude faster for providers which
support compiled expressions (eg Postgres), also allows algorithm
to take advantage of database indexes created on attribute.

For a 4 million point PostGIS layer (with an index on attribute)
BEFORE: cancelled after 20 mins
AFTER: ~2 seconds

Author: nyalldawson

python/plugins/processing/algs/qgis/SelectByAttribute.py

@@ -26,7 +26,7 @@
 __revision__ = '$Format:%H$'
 
 from PyQt4.QtCore import QVariant
-from qgis.core import QgsExpression
+from qgis.core import QgsExpression, QgsFeatureRequest
 from processing.core.GeoAlgorithm import GeoAlgorithm
 from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
 from processing.core.parameters import ParameterVector
@@ -115,18 +115,12 @@ def processAlgorithm(self, progress):
             raise GeoAlgorithmExecutionException(
                 self.tr('Unsupported field type "%s"' % fields[idx].typeName()))
 
-        expression = QgsExpression(expr)
-        expression.prepare(fields)
-
-        features = vector.features(layer)
-
-        selected = []
-        count = len(features)
-        total = 100.0 / float(count)
-        for count, f in enumerate(features):
-            if expression.evaluate(f, fields):
-                selected.append(f.id())
-            progress.setPercentage(int(count * total))
+        qExp = QgsExpression(expr)
+        if not qExp.hasParserError():
+            qReq = QgsFeatureRequest(qExp)
+        else:
+            raise GeoAlgorithmExecutionException(qExp.parserErrorString())
+        selected = [f.id() for f in layer.getFeatures(qReq)]
 
         layer.setSelectedFeatures(selected)
         self.setOutputValue(self.OUTPUT, fileName)