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Chapter 13: Problem 19

Discuss the techniques for record deletion.

Short Answer

Expert verified
There are four key techniques for record deletion. These include: Direct Deletion, Mark and Sweep, Reorganization, and Tombstone methods. Each method has its unique roles and use-cases.

Step by step solution

01

Direct Deletion

Direct deletion is the simplest method and typically used by database management systems. In this method, the specific record is directly removed from the database using a command like 'DELETE FROM table WHERE condition'. It is important to specify the correct condition to avoid accidental deletion of other records.
02

Mark and Sweep

In this technique, records are not directly deleted but marked for deletion. A 'sweep' operation could then be conducted during off-peak hours or at regular intervals to remove marked records. This method reduces the impact on performance during peak usage hours but needs extra storage space for marking records.
03

Reorganization

This technique involves completely reorganizing the data file. The records that are not to be deleted are written to a new file, then the old file is deleted and the new file renamed. Although this method is time-consuming, it efficiently clears deleted records and their residues.
04

Tombstone

In situations where synchronization between replicas is crucial, a 'tombstone' record might be used to indicate a record deletion. Instead of instantly deleting the record, it is replaced by a tombstone. The tombstone record is then propagated through the system, indicating to all replicas that they should also delete the corresponding record. Once all replicas have acknowledged the deletion, the tombstone can be removed.

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Most popular questions from this chapter

Why are disks, not tapes, used to store online database files?

A PARTS file with Part* as hash key includes records with the following Part* val. ues: 2369,3760,4692,4871,5659,1821,1074,7115,1620,2428,3943,4750 \(6975,4981,9208 .\) The file uses eight buckets, numbered 0 to 7. Each bucket is one disk block and holds two records. Load these records into the file in the given order, using the hash function \(h(K)=K \bmod 8 .\) Calculate the average number of block accesses for a random retrieval on Part#.

Consider a disk with the following characteristics (these are not parameters of any particular disk unit): block size \(B=512\) bytes; interblock gap size \(G=128\) bytes; number of blocks per track \(=20 ;\) number of tracks per surface \(=400 .\) A disk pack consists of 15 double-sided disks. a. What is the total capacity of a track, and what is its useful capacity (excluding interblock gaps)? b. How many cylinders are there? c. What are the total capacity and the useful capacity of a cylinder? d. What are the total capacity and the useful capacity of a disk pack? e. Suppose that the disk drive rotates the disk pack at a speed of 2400 rpm (revolutions per minute \() ;\) what are the transfer rate \((t r)\) in bytes/msec and the block transfer time \((b t t)\) in msec? What is the average rotational delay \((r d)\) in msec? What is the bulk transfer rate? (See Appendix B.) f. Suppose that the average seek time is 30 msec. How much time does it take (on the average) in msec to locate and transfer a single block, given its block address? g. Calculate the average time it would take to transfer 20 random blocks, and compare this with the time it would take to transfer 20 consecutive blocks using double buffering to save seek time and rotational delay.

Discuss the advantages and disadvantages of using (a) an unordered file, (b) an ordered file, and (c) a static hash file with buckets and chaining. Which operations can be performed efficiently on each of these organizations, and which operations are expensive?

What characterizes the levels in RAID organization?
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