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CS 338 (6)

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Computer Science
CS 338
Khuzaima Daudjee

Chapter 5Indexing File Systems and DisksDatabases are stored in files o 1 file per relation OR 1 file for entire databaseFiles reside on disksDBMS must transfer to buffer for access read modify update deleteData transferred between disk and memory in blocks slow process IndexingIndex consists of extra information data structure added to file to provide faster access to dataIndex is defined on one or more attributes of relationIndex fined on attribute A of relation R o Reduce execution time for selections that specify conditions involving A o Increase execution time for insertions and deletions of tuples from R o Increase size of file required to store R BtreesWidelyused index structuresFully dynamic easily grow and shrinkIndex blocksdata blocks half full o Index blocks stores maximum of m keys and m1 pointersEach block stores at least m2 keys and m21 pointers o Data blocks stores maximum of n rowsEach block stores at least n12 records contains 2 pointers Clustering vs Nonclustering IndexIndex on attribute A is a clusteringindex if tuples with similar values for A are stored together in same blockOther indices are nonclustering or secondary indicesRelation may have at most 1 clustering index and any number of nonclustering indices Index vs Sequential ScanUsing an index is not always the best way to answer range query o Depends on how many tuples the query will retrieve
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