revert 415faa12

that was mistakenly merged from mysql-5.5.47

sql/item_subselect.cc

@@ -1748,27 +1748,6 @@ Item_in_subselect::single_value_transformer(JOIN *join)
       runtime created Ref item which is deleted at the end
       of the statement. Thus one of 'substitution' arguments
       can be broken in case of PS.
-
-      @todo
-      Why do we use real_item()/substitutional_item() instead of the plain
-      left_expr?
-      Because left_expr might be a rollbackable item, and we fail to properly
-      rollback all copies of left_expr at end of execution, so we want to
-      avoid creating copies of left_expr as much as possible, so we use
-      real_item() instead.
-      Doing a proper rollback is difficult: the change was registered for the
-      original item which was the left argument of IN. Then this item was
-      copied to left_expr, which is copied below to substitution->args[0]. To
-      do a proper rollback, we would have to restore the content
-      of both copies as well as the original item. There might be more copies,
-      if AND items have been constructed.
-      The same applies to the right expression.
-      However, using real_item()/substitutional_item() brings its own
-      problems: for example, we lose information that the item is an outer
-      reference; the item can thus wrongly be considered for a Keyuse (causing
-      bug#17766653).
-      When WL#6570 removes the "rolling back" system, all
-      real_item()/substitutional_item() in this file should be removed.
     */ 
     substitution= func->create(left_expr, where_item);
     have_to_be_excluded= 1;
@@ -2055,9 +2034,6 @@ Item_in_subselect::create_single_in_to_exists_cond(JOIN *join,
   }
   else
   {
-    /*
-      Grep for "WL#6570" to see the relevant comment about real_item.
-    */
     Item *item= (Item*) select_lex->item_list.head()->real_item();
 
     if (select_lex->table_list.elements)

sql/sql_select.cc

@@ -4114,17 +4114,6 @@ add_key_field(JOIN *join,
               Field *field, bool eq_func, Item **value, uint num_values,
               table_map usable_tables, SARGABLE_PARAM **sargables)
 {
-  if (field->table->reginfo.join_tab == NULL)
-  {
-    /*
-       Due to a bug in IN-to-EXISTS (grep for real_item() in item_subselect.cc
-       for more info), an index over a field from an outer query might be
-       considered here, which is incorrect. Their query has been fully
-       optimized already so their reginfo.join_tab is NULL and we reject them.
-    */
-    return;
-  }
-
   uint optimize= 0;  
   if (eq_func &&
       ((join->is_allowed_hash_join_access() &&