1 files changed, 43 insertions, 0 deletions
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index b3c002b9eb..bc77705e97 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -639,6 +639,39 @@ class RoomMemberWorkerStore(EventsWorkerStore):
         count = yield self.runInteraction("did_forget_membership", f)
         defer.returnValue(count == 0)
 
+    @cached()
+    def get_forgotten_rooms_for_user(self, user_id):
+        """Gets all rooms the user has forgotten.
+
+        Args:
+            user_id (str)
+
+        Returns:
+            Deferred[set[str]]
+        """
+
+        def _get_forgotten_rooms_for_user_txn(txn):
+            # This is a slightly convoluted query that first looks up all rooms
+            # that the user has forgotten in the past, then rechecks that list
+            # to see if any have subsequently been updated. This is done so that
+            # we can use a partial index on `forgotten = 1` on the assumption
+            # that few users will actually forget many rooms.
+            sql = """
+                SELECT room_id, (
+                    SELECT count(*) FROM room_memberships
+                    WHERE room_id = m.room_id AND user_id = m.user_id AND forgotten = 0
+                ) AS count
+                FROM room_memberships AS m
+                WHERE user_id = ? AND forgotten = 1
+                GROUP BY room_id, user_id;
+            """
+            txn.execute(sql, (user_id,))
+            return set(row[0] for row in txn if row[1] == 0)
+
+        return self.runInteraction(
+            "get_forgotten_rooms_for_user", _get_forgotten_rooms_for_user_txn
+        )
+
     @defer.inlineCallbacks
     def get_rooms_user_has_been_in(self, user_id):
         """Get all rooms that the user has ever been in.
@@ -670,6 +703,13 @@ class RoomMemberStore(RoomMemberWorkerStore):
             _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME,
             self._background_current_state_membership,
         )
+        self.register_background_index_update(
+            "room_membership_forgotten_idx",
+            index_name="room_memberships_user_room_forgotten",
+            table="room_memberships",
+            columns=["user_id", "room_id"],
+            where_clause="forgotten = 1",
+        )
 
     def _store_room_members_txn(self, txn, events, backfilled):
         """Store a room member in the database.
@@ -771,6 +811,9 @@ class RoomMemberStore(RoomMemberWorkerStore):
             txn.execute(sql, (user_id, room_id))
 
             self._invalidate_cache_and_stream(txn, self.did_forget, (user_id, room_id))
+            self._invalidate_cache_and_stream(
+                txn, self.get_forgotten_rooms_for_user, (user_id,)
+            )
 
         return self.runInteraction("forget_membership", f)