Avoid unnecessary db query on index update

pks_seen is only needed if objects are removed from
index, so only compute it if necessary.
Improve pks_seen to not build an intermediary list.

Author: stefanw

haystack/management/commands/update_index.py

@@ -243,7 +243,6 @@ def update_backend(self, label, using):
             if self.verbosity >= 1:
                 print u"Indexing %d %s" % (total, force_unicode(model._meta.verbose_name_plural))
 
-            pks_seen = set([smart_str(pk) for pk in qs.values_list('pk', flat=True)])
             batch_size = self.batchsize or backend.batch_size
 
             if self.workers > 0:
@@ -267,8 +266,10 @@ def update_backend(self, label, using):
                     # They're using a reduced set, which may not incorporate
                     # all pks. Rebuild the list with everything.
                     qs = index.index_queryset().values_list('pk', flat=True)
-                    pks_seen = set([smart_str(pk) for pk in qs])
+                    pks_seen = set(smart_str(pk) for pk in qs)
                     total = len(pks_seen)
+                else:
+                    pks_seen = set(smart_str(pk) for pk in qs.values_list('pk', flat=True))
 
                 if self.workers > 0:
                     ghetto_queue = []