Re: Update statement flow

agarwalp_at_eeism.com wrote:
> I am not clear about the the redo logs, undo logs and in general what
> happens to an Update statement.

Have you read the concepts manual?

>
> from what i have read when a data is modified the modified data is
> stored in the undo log.

The unmodified data (the data as it was before modification) is stored in undo.

> If it is not comitted then other users read the
> data from the undo log.
> What happens when an update statement is never commited ?

Then it will be rolled back, eventually.

> From i have
> read that uncommitted data will be saved to the data file.

Perhaps. If the database block that holds the uncommitted data gets written back to the data file, then the uncommitted data will be saved to the data file. However, that row in the data file will have a flag set to show the row is (possibly) uncommited, just like it had when it was in memory.

> if the update is committed then it is updated in the datafile

Eventually. The commit wouldn't inspire the process to go update the datafile. Not until some process (perhaps the same process, but probably a different one) read that block back into memory, saw the "possibly uncommited" flag, checked on it to see it was in fact commited, did a block clean out; and then that block got written to disk again. At that point it would be updated in the datafile.

> as well
> the redo log files.

Redo logs get appended, they don't get updated.

> If it is in the arcchive mode as soon as the
> redolog file is full it is updated in thr archive file.

Archived log files also don't get updated. They just accumulate.

> If the update is in a transaction the updated uncommitted data is
> stored in the roll back segments.

All updates are in transcations. rollback is effectively the same as undo, so this question is the same as you first.

>
> Plz help me by providing a detail flow of what actually happens
> including all the process involved.

If I do your homework for you, do I get your grade?

Xho

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