From a68807d4260778cff706fc37ef1badd4fd0d8bf2 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 16 Sep 2016 11:34:01 +0100
Subject: [PATCH] Comment

---
 synapse/storage/event_federation.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index 53289f556..3d62451de 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -393,6 +393,8 @@ class EventFederationStore(SQLBaseStore):
 
     def _delete_old_forward_extrem_cache(self):
         def _delete_old_forward_extrem_cache_txn(txn):
+            # Delete entries older than a month, while making sure we don't delete
+            # the only entries for a room.
             sql = ("""
                 DELETE FROM stream_ordering_to_exterm
                 WHERE