Add a shortcut return when there are no events to purge.

changelog.d/19093.misc

@@ -0,0 +1 @@
+Add a shortcut return when there are no events to purge.

synapse/storage/databases/main/purge_events.py

@@ -239,10 +239,21 @@ def _purge_history_txn(
 
         txn.execute("SELECT event_id, should_delete FROM events_to_purge")
         event_rows = txn.fetchall()
+
+        deletable_events = sum(1 for e in event_rows if e[1])
+        if deletable_events == 0:
+            logger.info("[purge] no events found to delete")
+
+            # For the sake of cleanliness: drop the temp table.
+            # This will commit the txn in sqlite, so make sure to keep this actually last.
+            txn.execute("DROP TABLE events_to_purge")
+            # no referenced state groups
+            return set()
+
         logger.info(
             "[purge] found %i events before cutoff, of which %i can be deleted",
             len(event_rows),
-            sum(1 for e in event_rows if e[1]),
+            deletable_events,
         )
 
         logger.info("[purge] Finding new backward extremities")