Gan,

Unique indexes can be messed up by deleting a record, editing a cord so the index key changes (the edited record will affect the unique index but no other record will), and other things.

It is the very anture of a unique index that causes it to updated undependably and causes the4 requirement to reindex it every time you go to use it.

As I said before, don't use them. If you require a unique index for some operation, then create it, use it, and deleted it.