DEGRADATION


ISSUE:
------
This bug is in the function TABLE::refix_gc_items(). This
function will be called for each query that uses the TABLE
object. When the function calls fix_fields_gcol_func(), new
items might be created. In this case new Item_cache_real
objects are created. The items created by the call to
fix_fields_gcol_func() are created in the table's mem_root.
They are added to the field's gcol_info->item_free_list by
the following code:

      // We need append the new items to orignal item lists
        Item *item= vfield->gcol_info->item_free_list;
        while(item->next)
          item= item->next;
        item->next= gcol_arena.free_list;

The bug occurs due to the following:
-each time refix_gc_items() is called for this generated
 column expression, two new item objects are produced and
 added to the end of the item_free_list which is on table's
 mem_root.
-memory increase happens because item_free_list is growing.
-the performance drop happens due to spending time in the
 above while loop to find the end of the ever growing
 item_free_list.

The code that adds new items to the
gcol_info->item_free_list was added in the fix for
Bug#22392268. Before that fix, items added to the item tree
for the generated column expression as part of calling
TABLE::refix_gc_items() would be deleted when the statement
ended, since they were created on the THD's stmt_arena. If
the item was part of a permanent transformation, the next
time this TABLE object was used, we could access deleted
memory when using the item tree for the generated column
expression.

The fix for Bug#22392268 was to make sure that any created
items that are added to gcol_info->item_free_list have the
same life-span as the TABLE object. This worked fine for
generated column expressions where TABLE::refix_gc_items()
does permanent changes to the item tree but not in cases
where new items are produced on every call to
fix_fields_gcol_func().

SOLUTION:
---------
A new boolean flag permanent_changes_completed is added to
the Generated_column class. This flag is set only when the
permanent changes to the gcol function's item tree are
completed.

The fix for Bug#22392268 has been made subject to
permanent_changes_completed. We create objects on the
table's mem_root only when the permanent changes to the
gcol function's item tree haven't been completed.