diff options
Diffstat (limited to 'sql/item_subselect.cc')
| -rw-r--r-- | sql/item_subselect.cc | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc index eb1e1a3d9b7..9f508ad3f83 100644 --- a/sql/item_subselect.cc +++ b/sql/item_subselect.cc @@ -1782,6 +1782,27 @@ 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; @@ -2070,6 +2091,9 @@ 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) |