Indexed Sequential Access Method
Indexed sequential file; ISAM; ISAM file
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).
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 queries reference only a small...
- 2.Behymer JA, Ogilive RA, Merten AG. Analysis of indexed sequential and direct access file organizations. In: Proceedings of the 1974 ACM SIGFIDET (SIGMOD) Workshop on Data Description, Access and Control; 1974. p. 186–212.Google Scholar
- 3.IBM Corporation. IBM system/360 operating system data management services. 2nd ed. 1972. C26-3746-1.Google Scholar
- 7.Wong KF, Strauss JG. An analysis of ISAM performance improvement options. Manag Datamat J. 1975;4(3):95–107.Google Scholar