diff --git a/synapse/storage/data_stores/main/devices.py b/synapse/storage/data_stores/main/devices.py

index ee3a2ab031..536cef3abd 100644
--- a/synapse/storage/data_stores/main/devices.py
+++ b/synapse/storage/data_stores/main/devices.py
@@ -55,6 +55,10 @@ DROP_DEVICE_LIST_STREAMS_NON_UNIQUE_INDEXES = (
 
 BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES = "remove_dup_outbound_pokes"
 
+BG_UPDATE_DROP_DEVICE_LISTS_OUTBOUND_LAST_SUCCESS_NON_UNIQUE_IDX = (
+    "drop_device_lists_outbound_last_success_non_unique_idx"
+)
+
 
 class DeviceWorkerStore(SQLBaseStore):
     def get_device(self, user_id, device_id):
@@ -342,32 +346,23 @@ class DeviceWorkerStore(SQLBaseStore):
 
     def _mark_as_sent_devices_by_remote_txn(self, txn, destination, stream_id):
         # We update the device_lists_outbound_last_success with the successfully
-        # poked users. We do the join to see which users need to be inserted and
-        # which updated.
+        # poked users.
         sql = """
-            SELECT user_id, coalesce(max(o.stream_id), 0), (max(s.stream_id) IS NOT NULL)
+            SELECT user_id, coalesce(max(o.stream_id), 0)
             FROM device_lists_outbound_pokes as o
-            LEFT JOIN device_lists_outbound_last_success as s
-                USING (destination, user_id)
             WHERE destination = ? AND o.stream_id <= ?
             GROUP BY user_id
         """
         txn.execute(sql, (destination, stream_id))
         rows = txn.fetchall()
 
-        sql = """
-            UPDATE device_lists_outbound_last_success
-            SET stream_id = ?
-            WHERE destination = ? AND user_id = ?
-        """
-        txn.executemany(sql, ((row[1], destination, row[0]) for row in rows if row[2]))
-
-        sql = """
-            INSERT INTO device_lists_outbound_last_success
-            (destination, user_id, stream_id) VALUES (?, ?, ?)
-        """
-        txn.executemany(
-            sql, ((destination, row[0], row[1]) for row in rows if not row[2])
+        self.db.simple_upsert_many_txn(
+            txn=txn,
+            table="device_lists_outbound_last_success",
+            key_names=("destination", "user_id"),
+            key_values=((destination, user_id) for user_id, _ in rows),
+            value_names=("stream_id",),
+            value_values=((stream_id,) for _, stream_id in rows),
         )
 
         # Delete all sent outbound pokes
@@ -725,6 +720,21 @@ class DeviceBackgroundUpdateStore(SQLBaseStore):
             BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES, self._remove_duplicate_outbound_pokes,
         )
 
+        # create a unique index on device_lists_outbound_last_success
+        self.db.updates.register_background_index_update(
+            "device_lists_outbound_last_success_unique_idx",
+            index_name="device_lists_outbound_last_success_unique_idx",
+            table="device_lists_outbound_last_success",
+            columns=["destination", "user_id"],
+            unique=True,
+        )
+
+        # once that completes, we can remove the old non-unique index.
+        self.db.updates.register_background_update_handler(
+            BG_UPDATE_DROP_DEVICE_LISTS_OUTBOUND_LAST_SUCCESS_NON_UNIQUE_IDX,
+            self._drop_device_lists_outbound_last_success_non_unique_idx,
+        )
+
     @defer.inlineCallbacks
     def _drop_device_list_streams_non_unique_indexes(self, progress, batch_size):
         def f(conn):
@@ -799,6 +809,20 @@ class DeviceBackgroundUpdateStore(SQLBaseStore):
 
         return rows
 
+    async def _drop_device_lists_outbound_last_success_non_unique_idx(
+        self, progress, batch_size
+    ):
+        def f(txn):
+            txn.execute("DROP INDEX IF EXISTS device_lists_outbound_last_success_idx")
+
+        await self.db.runInteraction(
+            "drop_device_lists_outbound_last_success_non_unique_idx", f,
+        )
+        await self.db.updates._end_background_update(
+            BG_UPDATE_DROP_DEVICE_LISTS_OUTBOUND_LAST_SUCCESS_NON_UNIQUE_IDX
+        )
+        return 1
+
 
 class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
     def __init__(self, database: Database, db_conn, hs):