One other isolation level is supported by Oracle: you can declare that a session or
transaction has an isolation level of READ ONLY. As the name implies, this level
explicitly prohibits any write operations and provides an accurate view of all the data
at the time the transaction began.
Oracle Concurrency Features
Three features are used by Oracle to implement multiversion read consistency:
Rollback segments
Rollback segments are structures in the Oracle database that store ???undo??? information
for transactions in case of rollback. This information restores database
rows to the state they were in before the transaction in question started. When a
transaction starts changing some data in a block, it first writes the old image of
the data to a rollback segment. The information stored in a rollback segment
provides the information necessary to roll back a transaction and supports multiversion
read consistency.
A rollback segment is different from a redo log. The redo log is used to log all
transactions to the database and recover the database in the event of a system
failure, while the rollback segment provides rollback for transactions and read
consistency.
Pages:
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429