index is a required url part

rest-api-spec/src/main/resources/rest-api-spec/api/update_by_query.json

@@ -8,6 +8,7 @@
       "comment": "most things below this are just copied from search.json",
       "parts": {
         "index": {
+         "required" : true,
          "type" : "list",
          "description" : "A comma-separated list of index names to search; use `_all` or empty string to perform the operation on all indices"
         },