Editing ISAM (section)

{{Short description|Method for creating, maintaining, and manipulating computer files}}
{{other uses}}
{{More citations needed section|date=January 2014|talk=Anchor}}
'''Indexed Sequential Access Method''' ('''ISAM''') is a method for creating, maintaining, and manipulating [[computer file]]s of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are maintained to achieve fast retrieval of required file records in [[indexed file]]s. [[IBM]] originally developed ISAM for [[mainframe computer]]s, but implementations are available for most computer systems.

The term ''ISAM'' is used for several related concepts:
*The IBM ISAM product and the [[algorithm]] it employs.<ref>{{cite book | author=Chin, Y.H. | title=Proceedings of the 1st International Conference on Very Large Data Bases - VLDB '75 | chapter=Analysis of VSAM's free-space behavior | year=1975 | pages=514–515| doi=10.1145/1282480.1282529 | isbn=9781450318181 | s2cid=11082747 }}</ref>
*A [[database]] system where an application developer directly uses an [[application programming interface]] to search indexes in order to locate records in data files. In contrast, a [[relational database]] uses a [[query optimizer]] which automatically selects indexes.<ref>{{cite web | url=http://www.techrepublic.com/article/explore-the-differences-between-isam-and-relational-databases/ | title=Explore the differences between ISAM and relational databases | date=2004-02-13 | access-date=17 October 2014 | author=Bogue, Robert L.}}</ref>
*An indexing algorithm that allows both sequential and keyed access to data.<ref>{{cite journal | title=Analysis of index-sequential files with overflow chaining | author=Larson, Per-Åke | journal=ACM Transactions on Database Systems | year=1981 | volume=6 | issue=4| pages=671–680 | doi=10.1145/319628.319665 | s2cid=16261748 | doi-access=free }}</ref> Most databases use some variation of the [[B-tree]] for this purpose, although the original IBM ISAM and [[VSAM]] implementations did not do so.
*Most generally, any index for a database. Indexes are used by almost all databases.