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Physical Rowids

原创 IT综合 作者:tsinglee 时间:2007-11-22 14:05:37 0 删除 编辑

Physical rowids provide the fastest possible access to a row of a given table. They contain the
physical address of a row (down to the specific block) and allow you to retrieve the row in a single
block access. Oracle guarantees that as long as the row exists, its rowid does not change. These
performance and stability qualities make rowids useful for applications that select a set of rows,
perform some operations on them, and then access some of the selected rows again, perhaps with the
purpose of updating them.

Every row in a nonclustered table is assigned a unique rowid that corresponds to the physical
address of a row's row piece (or the initial row piece if the row is chained among multiple row
pieces). In the case of clustered tables, rows in different tables that are in the same data block
can have the same rowid.

A row's assigned rowid remains unchanged unless the row is exported and imported using the Import
and Export utilities. When you delete a row from a table and then commit the encompassing
transaction, the deleted row's associated rowid can be assigned to a row inserted in a subsequent
transaction.

A physical rowid datatype has one of two formats:

The extended rowid format supports tablespace-relative data block addresses and efficiently
identifies rows in partitioned tables and indexes as well as nonpartitioned tables and indexes.
Tables and indexes created by an Oracle8i (or higher) server always have extended rowids.
A restricted rowid format is also available for backward compatibility with applications developed
with Oracle7 or earlier releases.

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