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The use of locks during daily processing can be avoided if le modi cations, made due to updates, are not immediately re ected in the les When updating is deferred the blocks or records generated by update transactions are identi ed relative to the main database, but held on an update-log le The update-log le is merged into the database during periods of no or little simultaneous activity, typically at night Implicit in this approach is a disregard of issues of serializability: read operations do not re ect recent writes Deferred update can be necessary in systems where locks are restricted to large objects ( les) while update involves small objects (records or elds)
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There may also be opportunities to reorganize the granularity of a transaction to reduce the locking requirements The redundancy existing in les permits breaking up of some complex operations, and this reduces extensive locking requirements Transactions which access many records of a le should be analyzed for granularity In most systems the cost of claiming and releasing all le records individually is high peformance can b improved by locking of an entire le, but now sharing the le is made impossible; locking blocks may be optimal
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The critical section required to protect Jones in Fig 13-2 extends from the rst READ to the last REWRITE of the transaction, and the region to be locked involves two objects In complex queries the region which has to be locked can involve many objects: DEDUCT taxes FROM Employeespay AND PUT SUM INTO tax account Analysis shows that only one Employee record at a time and the one ledger record, namely tax account, has to be locked In order to reduce the size of the large locked object, the Employee le, locks are set and released within the loop Example 13-3 indicates the points with The number of Employee records speci ed by the DO-loop should be remain xed Setting a read-lock on no of employees will prevent insertion and deletion of entire Employee records, but other values in the le remain fully accessible during the Withhold tax transaction Example 13-3
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Withhold tax: TRANSACTION; tax account = 0; DO FOR no of employees; net salary = gross salary - tax; tax account = tax account + tax; END;
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We note that the need to lock is again related to the existence of redundant derived information The transitive functional dependency between an employee, tax, gross salary, and net salary is not obvious enough to be captured by a normalized view model Interference in access paths can cause transaction programs to fail Moving an object can cause a transaction trying to locate the object via pointer to pick up invalid data or, worse, garbage when further pointers are expected These problems can be minimized or entirely avoided by careful design of the procedures which manipulate the paths When access methods use indexes, obsoleted pointers can be kept safe by not reusing the space that they point to, although the space is freed when records are deleted or updated To avoid fetching bad data a tombstone is placed in the old spot The space becomes safely reusable when a reorganization can assure that all pointers to this spot are gone In Sec 3-4-1 we considered how locking for update protection can be minimized in B-trees In ring structures (Sec 3-6-1) the problems of losing a path through the database due to a simultaneous update become even more pronounced Figure 13-3 sketches a problem which can occur with any form of linked records If it is possible for one user to read the le while another user, or a system task, updates the chain,
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