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Is this statement true? If so is it better to use Guid or BIGINT?
It depends, but usually BIGINT is better.
Another question, related to keys... If a table uses Id as PK should it be clustered if BIGINT identity? Or if, say a customers table, has a CustomerCode that's used for searches should that be the column for a clustered index (even if when inserting the codes aren't sequential generating page moves in the database files)?
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It depends, but in this case, most likely your PK should be clustered and you will create non-clustered index on CustomerCode.