add comment

x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/querydsl/container/SearchHitFieldRef.java

@@ -87,6 +87,8 @@ private static boolean hasDocValues(DataType dataType) {
     }
 
     private static String format(DataType dataType) {
+        // We need epoch_millis for the tiebreaker timestamp field, because parsing timestamp strings
+        // can have a negative performance impact
         return dataType == DATETIME ? "epoch_millis" : null;
     }
 }