Export - CONSISTENT=Y
In export if CONSISTENT=Y then export of tables will be slower?
Pl. explain in detail which will be helpful to me.
you wrote it, you tell us why you think that
Originally Posted by bhattnirav
You have no clue about what a consistent snapshot of a relational database is, don't you?
Pablo (Paul) Berzukov
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Thanks DAVEY and PAVB.
I have asked because I have confusion.
may be this will hlp u to avoid the confusion in consistent snapshots ....
When you export a table, you are guaranteed that the contents of that table will be consistent with the time that the export of that table was started. This means that if you start exporting the table at 12:00 and someone makes changes to the data in the table at 12:05 and your export of this table finishes at 12:10, then the export will not contain any of the changes made between 12:00 and 12:10. You cannot change this behavior with Oracle's export utility.
The CONSISTENT parameter controls whether or not the entire export is consistent, even between tables. If CONSISTENT=N (the default), then the export of a table will be consistent, but changes can occur between tables. If CONSISTENT=Y, then the entire dump file is consistent with the point in time that you started the export.
and how does that answer his question?
Originally Posted by aru
Any updates on those tables would generate (and be stored in) rollback. Than applied after the table is released. That is what Oracle is saying
"If you use CONSISTENT=y and the volume of updates is large, the rollback segment usage will be large. In addition, the export of each table will be slower because the rollback segment must be scanned for uncommitted transactions."
If a consistent export is started then the export session has to read data as it was at the beginning of the export, regardless of any changes made. Therefore any changes made to the tables since the start of the export must be preserved in rollback segments until the export finishes.
An inconsistent export does a similar thing, but only for one table at a time
Aru you've got that backwards. Updates are applied to the table directly, but the old block is copied to rollback. A consistent export reads those rollback segments to get the consistent version of the tables / database. If the data expires out of rollback before you complete the export, you get a Snapshot Too Old error.
Rollback is not a queue for pending changes, it is storage for the data as it appeared before it was changed.
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