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    <p>The point about read consistency is this: suppose the customer rolls back their changes? Or suppose those changes fail because of a constraint violation or some system failure?</p> <p>Until the customer has successfully committed their changes those changes do not exist. Any decision you might make on the basis of a phantom read or a dirty read would have no more validity than the scenario you describe. Indeed they have less validity, because the changes are incomplete and hence inconsistent. Concrete example: if the customer's changes include making a deposit and making a withdrawal, how valid would your decision be if you had looked at the account when they had made the deposit but not yet made the withdrawal?</p> <p>Another example: a long running batch process updates the salary of every employee in the organisation. If you run a query against employees' salaries do you really want a report which shows you half the employees with updated salaries and half with their old salaries? </p> <p><strong>edit</strong></p> <p>Read consistency is achieved by using the information in the UNDO tablespace (rollback segments in the older implementation). When a session reads data from a table which is being changed by another session, Oracle retrieves the UNDO information which has been generated by that second session and substitutes it for the changed data in the result set presented to the first session.</p> <p>If the reading session is a long running query it might fail because due to the notorious <code>ORA-1555: snapshot too old</code>. This means the UNDO extent which contained the information necessary to assemble a read consistent view has been overwritten. </p> <p>Locks have nothing to do with read consistency. In Oracle writes don't block reads. The purpose of locks is to prevent other processes from attempting to change rows we are interested in. </p>
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    1. POOracle transaction read-consistency?
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    1. COOkay I got your point.It is about what happens if the transaction that has got lock fails? or what if the manager read live data before transaction is complete? By the way, do you know the internal mechanism of read consistency. Do locks come into play for ensuring read consistency? (I guess locks comes into play just to prevent concurrent transaction and that locks are not a must for read consistency).
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