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When a row in a table that does not have a clustering index is updated, and if the modified row no longer fits on the old page, SQL Server moves the row to a new page and leaves a forwarding pointer in the old page. As a result, the non-clustered indexes do not have to be modified to reflect the new row location, but will cause subsequent retrieval of this row, by means of non-clustered indexes to require an extra I/O.