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Семён Семёныч… Ошибся с языком, спасибо, поправил.
But let's assume that we want to add extra columns needed for a query to the unique index. Values of the new composite key may now fail to be unique


All the way around: If (book_ref) is unique, then (book_ref, book_date) can not be nonunique. What you are trying to say is the opposite: enforcing uniqueness of (book_ref, book_date) is not enough to get uniqueness of (book_ref) (which you want to use for foreign key).

Oh. Of course you're right, it was clearly a mistake. I've corrected the text, thank you.

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