The Indexed Sequential Access Method (ISAM) organizes records in key order and uses an index to support both direct retrieval by key and sequential processing. The index provides a route to a record; it is separate from the records themselves. ISAM is an access and file-organization method, not the name of a modern relational database product.
How indexed sequential access works
Records have an ordered place in a data area, typically based on a key. An index maps keys to locations, so software can use the index to find a particular record rather than scan every record. Because the records are also ordered, a program can process them sequentially.
IBM’s z/OS documentation describes one implementation in which QISAM builds track indexes and a cylinder index, with up to three levels of master indexes. In that documentation, QISAM supports creating or extending data, updating records in place, and retrieving records by sequential scan. BISAM documentation describes direct retrieval by key, in-place block updates, and insertion in key sequence. These are IBM-specific descriptions, not universal requirements for every system called ISAM. IBM z/OS DFSMS: Indexed sequential data sets
ISAM compared with HISAM and HIDAM
| Method | Meaning and organization | How access works |
|---|---|---|
| ISAM | Indexed Sequential Access Method; a general indexed-sequential file organization. | An index supports retrieval by key, while key-ordered records support sequential processing. |
| HISAM | Hierarchical Indexed Sequential Access Method; an IBM IMS database access method. | A unique sequence field in each root segment forms an index to root segments and database records. IBM says, “Unlike HSAM, however, each HISAM database record is indexed, allowing direct access to a database record.” IBM IMS: HISAM databases |
| HIDAM | Hierarchical Indexed Direct Access Method; an IBM IMS database access method. | Uses indexed access to roots in a hierarchical direct organization, with pointer access to subordinate segments. IBM IMS Glossary |
HISAM is not simply another spelling of generic ISAM: it is a specific method in IBM’s IMS hierarchy. For IMS database selection, IBM advises matching the database type to application processing requirements and the way most programs access the data; the method name alone does not establish which option will be fastest. IBM IMS: Analyzing data access
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What ISAM means on z/OS today
IBM’s z/OS DFSMS documentation states, “z/OS no longer supports indexed sequential (ISAM) data sets.” IBM recommends converting those data sets to key sequenced data sets (KSDS). This is specifically about indexed sequential data sets on z/OS; it does not establish that every historical or vendor-specific ISAM implementation has disappeared. IBM z/OS DFSMS: Indexed sequential data sets
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When the distinction matters
- For a keyed lookup, ask whether the access method provides an index or another direct route to the desired record.
- For ordered processing, consider whether records can be traversed sequentially in the required key order.
- For hierarchical data, distinguish an indexed-sequential arrangement such as HISAM from a hierarchical direct arrangement such as HIDAM.
- For legacy z/OS files, account for the platform’s lack of support for ISAM data sets and the documented KSDS conversion path.
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