A data file of 150,000 student-records is stored on disk with block size 4096 bytes. Records are sor

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GATE-CS is a competitive examination conducted in India. This question is from the 2021 GATE-CS Database Management Systems paper.

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This GATE-CS 2021 Database Management Systems question has a medium difficulty level and is marked as an important question.

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Question

A data file of 150,000 student-records is stored on disk with block size 4096 bytes. Records are sorted on RollNo (primary key). Record pointer size = 7 bytes. Each record has a candidate key attribute ANum of size 12 bytes. An index file with records of two fields (ANum value + record pointer) is built. Records of data and index files are not split across blocks. The number of blocks in the index file is ___

Accepted Answer

Answer: 698Concept -- Index File Block Calculation (from Ramez Elmasri and Shamkant Navathe, Fundamentals of Database Systems):Each index record = ANum (12 bytes) + record pointer (7 bytes) = 19 bytes.Index records per block = floor(4096 / 19) = floor(215.57...) = 215 records per block.Total index records = 150,000 (one per data record, since it is a dense index on ANum -- a non-ordering key).Number of index blocks = ceil(150,000 / 215) = ceil(697.67...) = 698 blocks.

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