Commited Changes!

Question

Hello All,
When we Commit, LGWR writes out the data(Vectors Changes) to the redo logfile and DBWR does nothing.
Say i've done millions of rows of updation and committed but actual datafile block is still not updated(delayed block clean out) and in the mean time one user queries the not updated data block then oracle will send it to the Undo block to look at the previous value.
How does oracle clears old value of undo to look at the new value from redo logfile?
Regards!

Answer 1

so where does user query get new values? in redo log? or buffer cache(how)? Let me take a chance to answer it.
Answer is redo log.
howCommitted data is written into the log files.Oracle makes sure that once committed the data is there in the redo log files which means from there it can recover at any point of time.
Redo log: The redo log is guaranteed to contain all committed data, including changes to UNDO segments. This is used as part of the database integrity. During normal database operation, the redo log is strictly write. Oracle never reads from redo during normal operation. This is for performance reasons.
Commit will be written to the redo log. So, a "commit marker" goes to redo, and is written to disk. This will mark the transaction committed in the undo segment header. Beyond that, nothing else has to happen immediately. Oracle is very good at deferring work till later. So, the blocks in the datafile still show as uncommitted, and point to the (now committed) transaction that's indicated in the undo segment header. The next time that block is read from disk, Oracle will do _"delayed block cleanout"_ and clean out those left over pointers. It will read the block header, see the pointers to the the transaction, go read the undo block header, find the transaction is committed, and update the datablock accordingly.
Source: Aman and Mark in different thread links.
Regards
Girish Sharma