Read Consistency & SCN

Read consistency is one of the prime conditions for RDBMS, under ACID property “I”, stand for isolation. The isolation property ensures that the concurrent execution of transactions result in a system state that would be obtained if transactions were executed serially, i.e. one after the other. Providing isolation is the main goal of concurrency control. Depending on concurrency control method, the effects of an incomplete transaction might not even be visible to another transaction.

Managing Read Consistency

Oracle Database automatically provides read consistency to a query so that all the data that the query sees comes from a single point in time (statement-level read consistency). Oracle Database can also provide read consistency to all of the queries in a transaction (transaction-level read consistency). Oracle Database uses the information maintained in its rollback segments to provide these consistent views. The rollback segments contain the old values of data that have been changed by uncommitted or recently committed transactions.

As a query enters the execution stage, the current system change number (SCN) is determined. As data blocks are read on behalf of the query, only blocks written with the observed SCN are used. Blocks with changed data (more recent SCNs) are reconstructed from data in the rollback segments, and the reconstructed data is returned for the query. Therefore, each query returns all committed data with respect to the SCN recorded at the time that query execution began. Changes of other transactions that occur during a query’s execution are not observed, guaranteeing that consistent data is returned for each query.
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