Fix Postgres query.

Author: clokep

synapse/storage/databases/main/relations.py

@@ -859,7 +859,7 @@ async def get_threads(
                 relation_type = '{RelationTypes.THREAD}'
                 {pagination_clause}
             GROUP BY relates_to_id
-            ORDER BY topological_ordering DESC, stream_ordering DESC
+            ORDER BY MAX(topological_ordering) DESC, MAX(stream_ordering) DESC
             LIMIT ?
         """