Re: Update statement flow

• Transaction starts
• before image of record goes to rollback segment. Rollback segment is in buffer cache.
• Modification of buffer cache block is recorded in redo log *buffer*
• Record is updated in buffer cache
• buffer cache modification is recorded in redo log *buffer*
• User issues commit, commit marker is recorded in redo log *buffer* and transaction info in data blocks in buffer cache is nullified.
• Oracle performs a checkpoint
• During checkpoint LGWR writes modifications of redo log *buffer* to online redo log including commit marker
• During checkpoint DBWR writes modifications of data buffers to data files.

Modified data is not stored in the undo log! Other users read the *before image* from the rollback segment or the undo segment. Information about this segment is recorded in the header of the modified data block.

Transactions will always end, either by commit, or by rollback. If data has already been written to the datafile, a rollback will simply restore the old situation from rollback/undo. If the transaction is uncommitted,other users will NEVER see the modifications!!!!!!

Please refer to the Oracle Concepts Manual for further clarification, and/or quote from this manual if you have further questions.

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Sybrand Bakker
Senior Oracle DBA