The simplest way to think of Oracle’s implementation of read consistency is to imagine each user operating a private copy of the database, hence the multiversion consistency model. Read Consistency, Undo Records, and Transactions To manage the multiversion consistency model, Oracle must create a read-consistent set of data when a table is queried (read) and simultaneously updated (written). When an update occurs, the original data values changed by the update are recorded in the database undo records. As long as this update remains part of an uncommitted transaction, any user that later queries the modified data views the original.
