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Export Parameter : Consistent

原创 IT综合 作者:tsinglee 时间:2007-09-07 15:33:58 0 删除 编辑

Specifies whether or not Export uses the SET TRANSACTION READ ONLY
statement to ensure that the data seen by Export is consistent to a single point in
time and does not change during the execution of theexp command. You should
specifyCONSISTENT=y when you anticipate that other applications will be
updating the target data after an export has started.
If you useCONSISTENT=n, each table is usually exported in a single transaction.
However, if a table contains nested tables, the outer table and each inner table are
exported as separate transactions. If a table is partitioned, each partition is exported
as a separate transaction.
Therefore, if nested tables and partitioned tables are being updated by other
applications, the data that is exported could be inconsistent. To minimize this
possibility, export those tables at a time when updates are not being done.

IfyouuseCONSISTENT=yandthevolumeofupdatesislarge,therollbacksegment
usage will be large. In addition, the export of each table will be slower because the
rollback segment must be scanned for uncommitted transactions.
Keep in mind the following points about usingCONSISTENT=y:
■ CONSISTENT=y is unsupported for exports that are performed when you are
connected as userSYS or you are usingAS SYSDBA, or both.
■ Export of certain metadata may require the use of theSYS schema within
recursive SQL. In such situations, the use ofCONSISTENT=y will be ignored.
Oracle Corporation recommends that you avoid making metadata changes
during an export process in whichCONSISTENT=y is selected.
■ To minimize the time and space required for such exports, you should export
tables that need to remain consistent separately from those that do not.
For example, export theemp and dept tables together in a consistent export,
and then export the remainder of the database in a second pass.
■ A "snapshot too old" error occurs when rollback space is used up, and space
taken up by committed transactions is reused for new transactions. Reusing
space in the rollback segment allows database integrity to be preserved with
minimum space requirements, but it imposes a limit on the amount of time that
a read-consistent image can be preserved.
If a committed transaction has been overwritten and the information is needed
for a read-consistent view of the database, a "snapshot too old" error results.
To avoid this error, you should minimize the time taken by a read-consistent
export. (Do this by restricting the number of objects exported and, if possible,
by reducing the database transaction rate.) Also, make the rollback segment as
large as possible.

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