Logging and Buffering - Transaction Processing - page 62

3.2 With physiological logging, what log records are produced by the transaction
of Problem 1.3 ? You may assume that no structure modifications occur and that no
other transactions are in progress.
3.3 At the highest level of abstraction, logical logging would mean writing original
SQL statements into the log, such as the single log record
n Wh T , update r set V D V C 1 where X > 50; m i
for the SQL statement
update r set V D V C 1 where X>50
done by transaction T . Are there any advantages in this approach? What are the
disadvantages?
3.4 With the no-steal buffering policy, no dirty (i.e., uncommitted) data never goes
to disk, because all the pages updated by a transaction are kept in the buffer until the
transaction has committed. Therefore, could we do without the before images in the
log records? Note that the before images are needed only to roll back uncommitted
transactions. Thus, a write operation WŒx; u ; v might be logged using a redo-only
log record that contains the after image v but not the before image u .
With the force buffering policy, a transaction is allowed to commit only after
all pages updated by it have been forced to disk. Does this policy allow a similar
simplification in what needs to be recorded in the log?
Is it possible for a database management system to apply both the no-steal and
force policies, at the same time?
3.5 Consider the extended read-write transaction model in which a transaction can
use the action CŒx to declare the updates on x done so far as committed before
the transaction terminates (see Problem 1.5 ). What log records must be generated
so that the write actions declared as committed will not be undone if the transaction
eventually aborts or performs a partial rollback?
3.6 What changes are needed to the logging protocol for our key-range transaction
model if the model is extended with the cursor mechanism outlined in Problem 1.7 ?
Bibliographical Notes
The fine-granular physiological logging protocol with redo-undo log records for
forward-rolling actions and redo-only compensation log records for undo actions in
partial or total rollbacks and with P AGE -LSN stamping presented in this chapter is—
apart from some minor simplifications—the one presented by Mohan et al. [ 1992a ]
for the ARIES recovery algorithm. The WA L and commit protocols are discussed
by Gray [ 1978 ], Gray et al. [ 1981 ], Mohan et al. [ 1992a ], and Gray and Reuter
[ 1993 ], among others. The steal-and-no-force buffering policy has been advocated
by Mohan et al. [ 1992a ] and Gray and Reuter [ 1993 ], among others.