M cheung william tan and carson woo sequential access

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Unformatted text preview: ccess Method Access Method • Indexed Sequential Access Method • Direct (Random) Access Method The way that data is physically stored affects the methods that can be used to access the data. 17 BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo Indexed Sequential Access Method (ISAM) (ISAM) Sequential Access Method Access Method • Data must be retrieved in the same In the the sequential storage environment, only this method can method can be used. 18 • Data are stored sequentially but individual physical sequence in which they are physical sequence in which they are stored records records can be accessed in any desired order by using an index. • Data are usually sorted by a key field are usually sorted by key field 122454 • Key field: a field used to uniquely identify a record from a group of records record from group of records Key Field 12346 134555 Index Magnetic Tape 12345 125555 122454 125555 134555 12347 One record Location Sector X, Track Y Sector P, Track Q Sector G, Track Z A table or list that relates record keys to physical locations on direct di access storage. Key fields BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 19 BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 20 Using Indexes Using Indexes Indexes (Good or Bad) (Good or Bad) • Advantages • A list of the key field of each record list of the key field of each record and its associated storage location – Non-sequential access: By using indexes, a record can be accessed directly. • An indexed field is a fiield in a fi f – Random...
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This document was uploaded on 03/04/2014 for the course COMM 335 at The University of British Columbia.

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