Sybase 12.4.2 - Manual (Page 323)

CHAPTER 8 Transactions and Versioning
303
The level of isolation that Adaptive Server IQ provides prevents several types
of inconsistencies. The ones most commonly encountered are listed here:
• Dirty Reads Transaction A modifies an object, but does not commit or
roll back the change. Transaction B reads the modified object. Then
Transaction A further changes the object before performing a
COMMIT . In
this situation, Transaction B has seen the object in a state that was never
committed.
• Non-Repeatable Reads Transaction A reads an object. Transaction B
then modifies or deletes the object and performs a COMMIT . If Transaction
A attempts to read the same object again, it will have been changed or
deleted.
• Phantom Data Elements Transaction A reads a set of data that satisfies
some condition. Transaction B then executes an INSERT and then a
COMMIT . The newly committed data now satisfies the condition, when it
did not previously. Transaction A then repeats the initial read and obtains
a different set of data.
• Lost Update In an application that uses cursors, Transaction A writes a
change for a set of data. Transaction B then saves an update that is based
on earlier data. Transaction A's changes are completely lost.
Adaptive Server IQ protects you from all of these inconsistencies by ensuring
that only one user can modify a table at any given time, by keeping the changes
invisible to other users until the changes are complete, and by maintaining
time-stamped snapshots of data objects in use at any time.
While IQ allows you to set the isolation level to 0, 1, 2, or 3 (comparable to
ANSI levels 1, 2, 3, or 4) using
SET OPTION ISOLATION_LEVEL , there is no
reason to do so. All users execute at isolation level 4, even if you set a different
level. There is no performance advantage to setting a lower isolation level.
Checkpoints, savepoints, and transaction rollback
Besides permitting concurrency, transaction processing plays an important role
in data recovery. Database recovery always recovers every committed
transaction. Transactions that have not committed at the time of a database
crash are not recovered.