What is rollback segment?

What is rollback segment?

Rating:

A rollback segment is an area in the memory of a computer, which stores data consistent to a particular instance of time. This process is helpful in providing read consistency to queries that were being issued before a change was made to the data segment associated with the rollback segment. User transactions can either be dynamically assigned to a rollback segment or users can explicitly assign a particular rollback segment to a data segment. In case rollback segments are dynamically assigned to transactions, Oracle assigns the next immediate segment that is available to the segment. The rollback segments are allocated as soon as the first DDL or DML statement is issued against the data segment. However, read-only transactions are not allocated rollback segments. A user can explicitly assign their transaction to a rollback segment that suits the requirements of his transaction. A transaction during its entire lifetime can only write the rollback data to the rollback segment assigned to it. After the completion of the transaction, the rollback data is released but not immediately destroyed. This is useful in providing read consistent data to queries that were issued before the transaction was committed. The rollback segments are written in a circular list fashion. After the last extent of the rollback segment is filled, Oracle starts to write the rollback entries to the first extent.

Rating: