ISAM: Indexed Sequential Access Method
What is it?
ISAM or Indexed Sequential Access Method provides reliable data storage with direct record access capability via file indexes. ISAM creates two files – the data file which stores the data as written and, the index file which contains the index values stored in index order with pointers back to the data record. Standard ISAM naming conventions use the extensions .idx for the index file and .dat for the data file, the exception to this is with MicroFocus Cobol ISAM which uses no extension for the data file.Indexes are defined as starting at a particular offset within the data structure for a specified length. Index values may be stored in ascending or descending order. There is no limit to the number of indexes per file and indexes may be added or removed at any point without affecting the integrity of the isam files.
ISAM makes use of a sophisticated b-tree search algorithm to store and retrieve index values, allowing for quick and direct access to the data. ISAM can start reading from any point within the file and can continue reading backward or forward through the index if an index order is specified or, backward or forward through the data file if sequential or record number order is specified, allowing for complete flexibility when accessing data.
DISAM96 Overview -->