exp CONSISTENT=Y 原理：export前发出SET TRANSACTION READ ONLY命令
今天才知道， exp CONSISTENT=Y 之所以能保持数据一致性，原来是在export前发出SET TRANSACTION READ ONLY命令，用来保持数据在同一个时间点。
metalink Note:113450.1
PURPOSE OF CONSISTENT=Y EXPORT
------------------------------
Export extracts data by issuing a series of SELECT statements and the snapshot time of each table's data corresponds to when the SELECT statement was issued for that table. If there is no database activity, then this is not an issue.
However, it is possible to manipulate the table after the export has started in which case the snapshot of the data may be a factor. Export does not get an exclusive lock on any table.
There is an option called CONSISTENT=Y. When this is enabled, EXPORT first issues a SET TRANSACTION READ ONLY command before doing the export. There is a risk of running out of rollback segment space and getting an "ORA-1555 snapshot too old (rollback segment too small)" error when doing a long running export.
metalink Note:113450.1
PURPOSE OF CONSISTENT=Y EXPORT
------------------------------
Export extracts data by issuing a series of SELECT statements and the snapshot time of each table's data corresponds to when the SELECT statement was issued for that table. If there is no database activity, then this is not an issue.
However, it is possible to manipulate the table after the export has started in which case the snapshot of the data may be a factor. Export does not get an exclusive lock on any table.
There is an option called CONSISTENT=Y. When this is enabled, EXPORT first issues a SET TRANSACTION READ ONLY command before doing the export. There is a risk of running out of rollback segment space and getting an "ORA-1555 snapshot too old (rollback segment too small)" error when doing a long running export.
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/35489/viewspace-1433038/ ，如需转载，请注明出处，否则将追究法律责任。
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