ISAM

The Indexed Sequential Access Method (ISAM) is a generic name of a set of algorithms used to enhance the access speed on computer files. These methods rely on duplicating the relevant data keys, creating indexes, that point to whole file records.

ISAM adds records to a file sequentially, so they are listed in the order they are added. This allows for sequential reading of the data as well. This, by itself would constitute simple SAM (Sequential Access Method), which was, in fact another system developed by IBM. ISAM adds an index to the file which allows for quick random access of any record simply by looking up some portion of the rcord in the index, just as a human might look up a page number in a book by looking for aword in the index at the back of the book.

ISAM was developed by IBM. A methodology called VSAM (Virtual Storage Access Method) was developed by IGM to replace it. Still later, IBM developed DB2, which, as of 2004, IBM promotes as their primary database management system. While DB2 is a relational system, ISAM antedates the concept of the relational database.