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    <p>No, on the contrary, it means you need to do a backup after a NOLOGGING load, not that you can't backup the database.</p> <p>Allow me to elaborate a bit. Normally, when you do DML in Oracle, the before images of the changes you are are making get logged in UNDO, and all the changes (including the UNDO changes) are first written to REDO. This is how Oracle manages transactions, instance recovery, and database recovery. If a transaction is aborted or rolled back, Oracle uses the information in UNDO to undo the changes your transaction made. If the instance crashes, then on instance restart, Oracle will use the information in REDO and UNDO to recover up to the last committed transaction. First, Oracle will read the REDO and roll forward, then, use UNDO to roll back all the transactions that were not committed at the time of the crash. In this way, Oracle is able to recover up to the last committed transaction. </p> <p>Now, when you specify an APPEND hint on an insert statement, Oracle will execute the INSERT with direct load. This means that data is loaded into brand new, never before used blocks, from above the highwater mark. Because the blocks being loaded are brand new, there is no "before image", so, Oracle can avoid writing UNDO, which improves performance. If the database is in NOARCHIVELOG mode, then Oracle will also not write REDO. On a database in ARCHIVELOG mode, Oracle will still write REDO, unless, before you do the insert /*+ append */, you set the table to NOLOGGING, (i.e. alter table tab_name nologging;). In that case, REDO logging is disabled for the table. However, this is where you could run into backup/recovery implications. If you do a NOLOGGING direct load, and then you suffer a media failure, <em>and</em> the datafile containing the segment with the nologging operation is restored from a backup taken before the nologging load, then the redo log will <em>not</em> contain the changes required to recover that segment. So, what happens? Well, when you do a NOLOGGING load, Oracle writes extent invaldation records to the redo log, instead of the actual changes. Then, if you use that redo in recovery, those data blocks will be marked logically corrupt. Any subsequent queries against that segment will get an ORA-26040 error.</p> <p>So, how to avoid this? Well, you should always take a backup imediately following any NOLOGGING direct load. If you restore/recover from a backup taken <em>after</em> the nologging load, there is no problem, because the data will be in the datablocks in the file that was restored.</p> <p>Hope that's clear,</p> <p>-Mark</p>
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    1. USMark J. Bobak
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