Oracle Essentials: Oracle9i, Oracle8i and Oracle8, Second Edition

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.

Blocks of rollback segments are cached in the System Global Area just like blocks of tables and indexes. If rollback segment blocks are unused for a period of time, they may be aged out of the cache and written to disk.

System Change Number (SCN)

To preserve the integrity of the data in the database, it’s critical to keep track of the order in which actions were performed. Oracle must preserve the ordering of transactions with respect to time. The mechanism Oracle uses is the System Change Number.

The SCN is a logical timestamp that tracks the order in which events occurred. Oracle uses the SCN information in the ...