From f91faf09b30bf3d6b2997d3a147df23d4460b7fa Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 20 Jul 2016 10:18:09 +0100
Subject: [PATCH] Comment

---
 synapse/storage/stream.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/synapse/storage/stream.py b/synapse/storage/stream.py
index 3516636dd..862c5c3ea 100644
--- a/synapse/storage/stream.py
+++ b/synapse/storage/stream.py
@@ -96,6 +96,10 @@ def upper_bound(token, engine, inclusive=True):
 
 
 def filter_to_clause(event_filter):
+    # NB: This may create SQL clauses that don't optimise well (and we don't
+    # have indices on all possible clauses). E.g. it may create
+    # "room_id == X AND room_id != X", which postgres doesn't optimise.
+
     if not event_filter:
         return "", []