Use FilterExpression request for get_feature expression function

Makes the function magnitudes faster for Postgres layers where
the expression is compiled and can take advantage of Postgres
indexes on the layer.

Author: nyalldawson

src/core/qgsexpression.cpp

@@ -2033,20 +2033,18 @@ static QVariant fcnGetFeature( const QVariantList& values, const QgsExpressionCo
 
   const QVariant& attVal = values.at( 2 );
   QgsFeatureRequest req;
+  req.setFilterExpression( QString( "%1=%2" ).arg( QgsExpression::quotedColumnRef( attribute ),
+                                                   QgsExpression::quotedString( attVal.toString() ) ) );
   if ( !parent->needsGeometry() )
   {
     req.setFlags( QgsFeatureRequest::NoGeometry );
   }
   QgsFeatureIterator fIt = vl->getFeatures( req );
 
   QgsFeature fet;
-  while ( fIt.nextFeature( fet ) )
-  {
-    if ( fet.attribute( attributeId ) == attVal )
-    {
-      return QVariant::fromValue( fet );
-    }
-  }
+  if ( fIt.nextFeature( fet ) )
+    return QVariant::fromValue( fet );
+
   return QVariant();
 }