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The following explanation is given in this Microsoft Technical Article: Why does the first index on a view have to be CLUSTERED and UNIQUE? It must be UNIQUE to allow easy lookup of records in the view by key value during indexed view maintenance, and to prevent creation of views with duplicates, which would require special logic to maintain. It must ...


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In SQL Server all index keys must internally be unique. This is required to obtain lock keys that address exactly one row. It is also required for index maintenance. Imagine an NCI on a column that only has one value in it (100% duplicates). If a row is deleted from the table the storage engine must find the corresponding NCI row and delete it as well. If ...



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