diff --git a/synapse/storage/schema/delta/37/remove_auth_idx.py b/synapse/storage/schema/delta/37/remove_auth_idx.py
index 784f3b348..20ad8bd5a 100644
--- a/synapse/storage/schema/delta/37/remove_auth_idx.py
+++ b/synapse/storage/schema/delta/37/remove_auth_idx.py
@@ -36,6 +36,10 @@ DROP INDEX IF EXISTS transactions_have_ref;
 -- and is used incredibly rarely.
 DROP INDEX IF EXISTS events_order_topo_stream_room;
 
+-- an equivalent index to this actually gets re-created in delta 41, because it
+-- turned out that deleting it wasn't a great plan :/. In any case, let's
+-- delete it here, and delta 41 will create a new one with an added UNIQUE
+-- constraint
 DROP INDEX IF EXISTS event_search_ev_idx;
 """