diff --git a/mercurial/repoview.py b/mercurial/repoview.py
--- a/mercurial/repoview.py
+++ b/mercurial/repoview.py
@@ -94,6 +94,11 @@ def computeimpactable(repo):
     return frozenset(xrange(firstmutable, len(cl)))
 
 # function to compute filtered set
+#
+# When addding a new filter you MUST update the table at:
+#     mercurial.branchmap.subsettable
+# Otherwise your filter will have to recompute all its branches cache
+# from scratch (very slow).
 filtertable = {'visible': computehidden,
                'served': computeunserved,
                'immutable':  computemutable,