Oracle Default Storage Clause

The default storage clause generally applies to tablespaces and datafiles (even databases as a whole) where all objects and structures within the tablespace (datafile or database) can inherit the tablespace's default storage clause or override the tablespace default storage clause with it's own default storage clause.

Tablespaces are subject to fragmentation. Fragmentation is the main cause for space related problems with tablespaces. As tables are created and dropped rollback segments grow and are shrunk back to OPTIMAL or indexes are reorganised or rebuilt. All of these events result in the allocation or dropping of extents. Frequent deallocation of extents results in fragmented tablespaces. The ALTER command will allow coalescence of contiguous free space areas. Contiguous free space areas are deallocated extents which reside next to each other. Note that only rollback segments are shrunk automatically using the OPTIMAL setting.