diff --git a/core/common/models.py b/core/common/models.py
index 527f76ca..de6aa7dd 100644
--- a/core/common/models.py
+++ b/core/common/models.py
@@ -505,12 +505,13 @@ def index_batch(ids):
             batch_size = INDEX_BATCH_SIZE
             start = 0
             id_qs = queryset.order_by('-id').values_list('id', flat=True)
-            while True:
-                batch = list(id_qs[start:start + batch_size])
-                if not batch:
-                    break
+            # Keyset paging, so each page costs the same; a union can't be filtered, so it pages by offset
+            keyset = not id_qs.query.combinator
+            batch = list(id_qs[start:start + batch_size])
+            while batch:
                 run.attempt(start, batch, index_batch)
                 start += batch_size
+                batch = list(id_qs.filter(id__lt=batch[-1])[:batch_size] if keyset else id_qs[start:start + batch_size])
 
         if refresh:
             run.refresh(doc)
diff --git a/core/common/tasks.py b/core/common/tasks.py
index aacccd26..d3a02976 100644
--- a/core/common/tasks.py
+++ b/core/common/tasks.py
@@ -878,12 +878,14 @@ def index_in_parts(*parts):
 
 def exclude_source_members(queryset, source_id):
     """
-    Leaves out the queryset's rows that are members of source_id. An explicit id__in: Django's exclude(sources=...)
-    correlates on the join table's own row id rather than the resource id, so it leaves the wrong rows out.
+    Leaves out the queryset's rows that are members of source_id. Correlated explicitly on the resource id:
+    Django's exclude(sources=...) correlates on the join table's own row id, so it leaves the wrong rows out.
+    NOT EXISTS, not NOT IN: Postgres can't turn NOT IN into an anti-join, and once the member list outgrows
+    work_mem it rescans that list for every row (hours on a large release).
     """
     through = queryset.model.sources.through
     member_field = queryset.model._meta.model_name + '_id'  # pylint: disable=protected-access
-    return queryset.exclude(id__in=through.objects.filter(source_id=source_id).values(member_field))
+    return queryset.filter(~Exists(through.objects.filter(source_id=source_id, **{member_field: OuterRef('id')})))
 
 
 def get_concepts_to_index(source, locales=None, exclude_locale=None, exclude_members_of=None):
diff --git a/core/common/tests.py b/core/common/tests.py
index deb2f9ad..f9ceb121 100644
--- a/core/common/tests.py
+++ b/core/common/tests.py
@@ -1528,7 +1528,9 @@ def test_batch_index_full_refresh_error_does_not_fail_the_run(self, _):
     @patch('core.common.models.parallel_bulk', return_value=[])
     def test_batch_index_partial_by_ids_refreshes_once_after_the_last_batch(self, parallel_bulk_mock):
         id_queryset = MagicMock()
-        id_queryset.__getitem__.side_effect = lambda batch_slice: [1, 2] if batch_slice.start == 0 else []
+        id_queryset.query.combinator = None
+        id_queryset.__getitem__.return_value = [2, 1]
+        id_queryset.filter.return_value.__getitem__.return_value = []
         queryset = Mock()
         queryset.order_by.return_value.values_list.return_value = id_queryset
         doc_instance = Mock()
@@ -1540,6 +1542,34 @@ def test_batch_index_partial_by_ids_refreshes_once_after_the_last_batch(self, pa
 
         self.assertNotIn('refresh', parallel_bulk_mock.call_args[1])
         doc_instance._index.refresh.assert_called_once_with()  # pylint: disable=protected-access
+        id_queryset.filter.assert_called_once_with(id__lt=1)
+
+    def _paged_ids(self, queryset):
+        pages = []
+
+        def get_actions(ids):
+            pages.append(list(ids))
+            return iter([])
+
+        with override_settings(TEST_MODE=False), patch('core.common.models.INDEX_BATCH_SIZE', 2), \
+                patch('core.common.models.BatchIndexRun.bulk'):
+            BaseModel.batch_index_partial_by_ids(queryset, ConceptDocument, get_actions, refresh=False)
+        return pages
+
+    def test_batch_index_partial_by_ids_pages_by_keyset(self):
+        source = OrganizationSourceFactory()
+        concepts = [ConceptFactory(parent=source) for _ in range(5)]
+        ids = sorted((concept.id for concept in concepts), reverse=True)
+
+        self.assertEqual(self._paged_ids(Concept.objects.filter(id__in=ids)), [ids[0:2], ids[2:4], ids[4:]])
+
+    def test_batch_index_partial_by_ids_pages_a_union_by_offset(self):
+        source = OrganizationSourceFactory()
+        concepts = [ConceptFactory(parent=source) for _ in range(5)]
+        ids = sorted((concept.id for concept in concepts), reverse=True)
+        queryset = Concept.objects.filter(id__in=ids[:3]).union(Concept.objects.filter(id__in=ids[3:]))
+
+        self.assertEqual(self._paged_ids(queryset), [ids[0:2], ids[2:4], ids[4:]])
 
     @override_settings(TEST_MODE=False)
     def test_batch_index_full_continues_remaining_batches_after_error(self):