Reference Work Entry

Encyclopedia of Database Systems

pp 1435-1438

Indexed Sequential Access Method

  • Alex DelisAffiliated withCollege of Computing, Georgia Institute of TechnologyUniversity of Athens
  • , Vassilis J. TsotrasAffiliated withDatabase Research Group David R. Cheriton School of Computer Science, University of WaterlooUniversity of California-Riverside


Indexed sequential file; ISAM file; ISAM


An indexed sequential access method is a static, hierarchical, disk-based index structure that enables both (single-dimensional) range and membership queries on an ordered data file. The records of the data file are stored in sequential order according to some data attribute(s). Since ISAM is static, it does not change its structure if records are added or deleted from the data file. Should new records be inserted into the data file, they are stored in an overflow area. Deleted records are removed from the file (leaving empty space).

Historical Background

Although transparent for the user of a DBMS, access methods play a key role in database performance. A major performance goal of a DBMS is to minimize the number of I/Os (i.e., blocks or pages transferred) between the disk and main memory. One way to achieve this goal is to minimize the number of I/Os when answering queries. Note that many q ...

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