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Determine the Number and Size of Extents (13)

原创 数据库开发技术 作者:tsinglee 时间:2007-09-08 14:40:51 0 删除 编辑

A tablespace that manages its extents locally can have either uniform extent sizes or
variable extent sizes that are determined automatically by the system. When you
create the tablespace, the UNIFORM or AUTOALLOCATE (system-managed) clause
specifies the type of allocation.
■ For uniform extents, you can specify an extent size or use the default size, which is
1 MB. Ensure that each extent contains at least five database blocks, given the
database block size. Temporary tablespaces that manage their extents locally can
only use this type of allocation.
■ For system-managed extents, Oracle determines the optimal size of additional
extents, with a minimum extent size of 64 KB. If the tablespaces are created with 'segment space management auto’, and if the database block size is 16K or higher, then Oracle manages segment size by creating extents with a minimum size of 1M.This is the default for permanent tablespaces.

The storage parameters INITIAL, NEXT, PCTINCREASE, and MINEXTENTS cannot be
specified at the tablespace level for locally managed tablespaces. They can, however,
be specified at the segment level. In this case, INITIAL, NEXT, PCTINCREASE, and
MINEXTENTS are used together to compute the initial size of the segment. After the
segment size is computed, internal algorithms determine the size of each extent.


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