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When a view/derived table is converted from merged to materialized the
items from the used_item lists are substituted for items referring to
the fields of the result of the materialization. The problem appeared
with queries employing natural joins. Since the resolution of a natural
join was performed only once the used_item list formed at the second
execution of the query lacked the references to the fields that were
used only in the equality predicates generated for the natural join.