Re: Isolation level and Locks

philippe wrote in message >
>Am I right to say that Oracle uses Rollback segment to prevent a user to
>see uncommitted data vs SQL Server which uses Locks on those data ??
yes, the data is changed in the data blocks rightaway, the old values are 'saved' in the rollback segments.

>
>To my mind, SQL Server has no Rollback segment and uses its log journal
>to rollback a transaction. To the opposite, Oracle uses rollback
>segments to undo a trnasaction without using the log journal (redo log)
>for an online session.

Oracle's redo log contains all the changes of any blocks, data as well as rollback blocks. i.e. if you change a value, the old value is saved in a rollback block (as part of a rollback segment). The new value is saved in the original data block. These block changes are mentioned in the redo log buffer, which will result in one or more redo log files.

>
>What is the default isolation level for oracle ?
I don't know exactly what you mean with 'isolation level', but if you mean the locking mechanism Oracle uses, it's row level. Only the rows you change are locked, anyone reading these rows will get the old values via the mentioned rollback segments, if some other session made a change without a commit.
>

Hope this helps

Jac.
jbeekers_at_xs4all.nl Received on Fri Apr 24 1998 - 17:39:16 CDT