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Interleaving occurs when an index extent, which is a group of eight index pages, is not physically contiguous because an extent for another index is intermingled with it. This condition can happen even when there is no logical fragmentation in the index. Although the pages may be physically and logically ordered, they are not necessarily contiguous. Switching between extents can impact performance as data access is inefficient. To resolve this issue, use SQL Defrag Manager to rebuild the indexes to store them in contiguous pages and reduce the need to switch between extents.