diff --git a/contrib/revsetbenchmarks.txt b/contrib/revsetbenchmarks.txt
--- a/contrib/revsetbenchmarks.txt
+++ b/contrib/revsetbenchmarks.txt
@@ -12,4 +12,5 @@ max(tip:0)
 min(0:tip)
 0::
 min(0::)
+roots((tip~100::) - (tip~100::tip))
 ::p1(p1(tip))::
diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -661,8 +661,18 @@ def _descendants(repo, subset, x, follow
     if not args:
         return baseset([])
     s = _revdescendants(repo, args, followfirst)
-    a = set(args)
-    return subset.filter(lambda r: r in s or r in a)
+
+    # Both sets need to be ascending in order to lazily return the union
+    # in the correct order.
+    args.ascending()
+
+    subsetset = subset.set()
+    result = (orderedlazyset(s, subsetset.__contains__, ascending=True) +
+              orderedlazyset(args, subsetset.__contains__, ascending=True))
+
+    # Wrap result in a lazyset since it's an _addset, which doesn't implement
+    # all the necessary functions to be consumed by callers.
+    return orderedlazyset(result, lambda r: True, ascending=True)
 
 def descendants(repo, subset, x):
     """``descendants(set)``
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -474,16 +474,6 @@ min: empty on unordered set
   2
   1
   0
-  $ log '1:: and reverse(all())'
-  9
-  8
-  7
-  6
-  5
-  4
-  3
-  2
-  1
   $ log 'rev(5)'
   5
   $ log 'sort(limit(reverse(all()), 3))'