purge_history: fix index use

event_push_actions doesn't have an index on event_id, so we need to specify
room_id.

synapse/storage/events.py

@@ -2314,6 +2314,20 @@ class EventsStore(SQLBaseStore):
                 ")" % (table,),
             )
 
+        # event_push_actions lacks an index on event_id, and has one on
+        # (room_id, event_id) instead.
+        for table in (
+            "event_push_actions",
+        ):
+            logger.info("[purge] removing events from %s", table)
+
+            txn.execute(
+                "DELETE FROM %s WHERE room_id = ? AND event_id IN ("
+                "    SELECT event_id FROM events_to_purge WHERE should_delete"
+                ")" % (table,),
+                (room_id, )
+            )
+
         # Mark all state and own events as outliers
         logger.info("[purge] marking remaining events as outliers")
         txn.execute(