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Extents in Oracle

An extent is a part of a segment. It consists of one or more contiguos db blocks.
Space for data on a harddisk is allocated in extents. An extent belongs to a tablespace.
One or more extents make a segment.
sys.fet$ and sys.uet$ maintaint free and used extents.

Thanks

Thanks to Rahul Agarwal who improved this page.
Thanks also to Eliza who notified me of a type on this page.