diff --git a/sql/sql_explain.h b/sql/sql_explain.h
index 2847fe4a5e1..919d51aef8b 100644
--- a/sql/sql_explain.h
+++ b/sql/sql_explain.h
@@ -338,7 +338,7 @@ extern const char *pushed_derived_text;
 extern const char *pushed_select_text;
 
 /*
-  Explain structure for a UNION.
+  Explain structure for a UNION [ALL].
 
   A UNION may or may not have "Using filesort".
 */