2 files changed, 38 insertions, 9 deletions
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 2695975a16..a1c289b24a 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -739,17 +739,11 @@ class EventCreationHandler(object):
                 "Attempting to create an event with %i prev_events"
                 % (len(prev_events_and_hashes),)
             )
+            prev_event_ids = [event_id for event_id, _, _ in prev_events_and_hashes]
         else:
-            prev_events_and_hashes = yield self.store.get_prev_events_and_hashes_for_room(
-                builder.room_id
-            )
-
-        prev_events = [
-            (event_id, prev_hashes)
-            for event_id, prev_hashes, _ in prev_events_and_hashes
-        ]
+            prev_event_ids = yield self.store.get_prev_events_for_room(builder.room_id)
 
-        event = yield builder.build(prev_event_ids=[p for p, _ in prev_events])
+        event = yield builder.build(prev_event_ids=prev_event_ids)
         context = yield self.state.compute_event_context(event)
         if requester:
             context.app_service = requester.app_service
diff --git a/synapse/storage/data_stores/main/event_federation.py b/synapse/storage/data_stores/main/event_federation.py
index 266fc9715f..88e6489576 100644
--- a/synapse/storage/data_stores/main/event_federation.py
+++ b/synapse/storage/data_stores/main/event_federation.py
@@ -177,6 +177,41 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
 
         return res
 
+    def get_prev_events_for_room(self, room_id: str):
+        """
+        Gets a subset of the current forward extremities in the given room.
+
+        Limits the result to 10 extremities, so that we can avoid creating
+        events which refer to hundreds of prev_events.
+
+        Args:
+            room_id (str): room_id
+
+        Returns:
+            Deferred[List[str]]: the event ids of the forward extremites
+
+        """
+
+        return self.db.runInteraction(
+            "get_prev_events_for_room", self._get_prev_events_for_room_txn, room_id
+        )
+
+    def _get_prev_events_for_room_txn(self, txn, room_id: str):
+        # we just use the 10 newest events. Older events will become
+        # prev_events of future events.
+
+        sql = """
+            SELECT e.event_id FROM event_forward_extremities AS f
+            INNER JOIN events AS e USING (event_id)
+            WHERE f.room_id = ?
+            ORDER BY e.depth DESC
+            LIMIT 10
+        """
+
+        txn.execute(sql, (room_id,))
+
+        return [row[0] for row in txn]
+
     def get_latest_event_ids_and_hashes_in_room(self, room_id):
         """
         Gets the current forward extremities in the given room