[memory] Optimise iteration, avoid multiple map lookups and an unnecessary expression evaluation

Author: nyalldawson

src/core/providers/memory/qgsmemoryfeatureiterator.cpp

@@ -108,14 +108,16 @@ bool QgsMemoryFeatureIterator::nextFeatureUsingList( QgsFeature &feature )
   bool hasFeature = false;
 
   // option 1: we have a list of features to traverse
+  QgsFeature candidate;
   while ( mFeatureIdListIterator != mFeatureIdList.constEnd() )
   {
+    candidate = mSource->mFeatures.value( *mFeatureIdListIterator );
     if ( !mFilterRect.isNull() )
     {
       if ( mRequest.flags() & QgsFeatureRequest::ExactIntersect )
       {
         // do exact check in case we're doing intersection
-        if ( mSource->mFeatures.value( *mFeatureIdListIterator ).hasGeometry() && mSelectRectEngine->intersects( mSource->mFeatures.value( *mFeatureIdListIterator ).geometry().constGet() ) )
+        if ( candidate.hasGeometry() && mSelectRectEngine->intersects( candidate.geometry().constGet() ) )
           hasFeature = true;
       }
       else if ( mSource->mSpatialIndex )
@@ -126,16 +128,16 @@ bool QgsMemoryFeatureIterator::nextFeatureUsingList( QgsFeature &feature )
       else
       {
         // do bounding box check if we aren't using a spatial index
-        if ( mSource->mFeatures.value( *mFeatureIdListIterator ).hasGeometry() && mSource->mFeatures.value( *mFeatureIdListIterator ).geometry().boundingBoxIntersects( mFilterRect ) )
+        if ( candidate.hasGeometry() && candidate.geometry().boundingBoxIntersects( mFilterRect ) )
           hasFeature = true;
       }
     }
     else
       hasFeature = true;
 
-    if ( mSubsetExpression )
+    if ( hasFeature && mSubsetExpression )
     {
-      mSource->mExpressionContext.setFeature( mSource->mFeatures.value( *mFeatureIdListIterator ) );
+      mSource->mExpressionContext.setFeature( candidate );
       if ( !mSubsetExpression->evaluate( &mSource->mExpressionContext ).toBool() )
         hasFeature = false;
     }
@@ -149,7 +151,7 @@ bool QgsMemoryFeatureIterator::nextFeatureUsingList( QgsFeature &feature )
   // copy feature
   if ( hasFeature )
   {
-    feature = mSource->mFeatures.value( *mFeatureIdListIterator );
+    feature = candidate;
     ++mFeatureIdListIterator;
   }
   else