Hide the encodesortkey expression to the user

src/sql/engine/expr/ob_expr_encode_sortkey.cpp

@@ -29,7 +29,7 @@ namespace sql
 ObExprEncodeSortkey::ObExprEncodeSortkey(ObIAllocator &alloc)
   : ObExprOperator(
       alloc, T_FUN_SYS_ENCODE_SORTKEY, N_ENCODE_SORTKEY, MORE_THAN_TWO, NOT_ROW_DIMENSION,
-      false, false)
+      INTERNAL_IN_MYSQL_MODE, INTERNAL_IN_ORACLE_MODE)
 {}
 
 ObExprEncodeSortkey::~ObExprEncodeSortkey()