From 0752ab7a3621b90073f9332fbfdc8afe16a3be01 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 4 Mar 2022 17:57:27 +0000
Subject: [PATCH] Reduce to-device queries for /sync. (#12163)

---
 changelog.d/12163.misc                        | 1 +
 synapse/storage/databases/main/deviceinbox.py | 3 +++
 2 files changed, 4 insertions(+)
 create mode 100644 changelog.d/12163.misc

diff --git a/changelog.d/12163.misc b/changelog.d/12163.misc
new file mode 100644
index 000000000..13de0895f
--- /dev/null
+++ b/changelog.d/12163.misc
@@ -0,0 +1 @@
+Reduce number of DB queries made during processing of `/sync`.
diff --git a/synapse/storage/databases/main/deviceinbox.py b/synapse/storage/databases/main/deviceinbox.py
index 1392363de..b4a1b041b 100644
--- a/synapse/storage/databases/main/deviceinbox.py
+++ b/synapse/storage/databases/main/deviceinbox.py
@@ -298,6 +298,9 @@ class DeviceInboxWorkerStore(SQLBaseStore):
                 # This user has new messages sent to them. Query messages for them
                 user_ids_to_query.add(user_id)
 
+        if not user_ids_to_query:
+            return {}, to_stream_id
+
         def get_device_messages_txn(txn: LoggingTransaction):
             # Build a query to select messages from any of the given devices that
             # are between the given stream id bounds.