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2. Various Keyed ISAM DBMS Systems

No, ISAM does not (at least here) stand for The International Society for Aerosols in Medicine. The Webopedia defines ISAM as an

Abbreviation for Indexed Sequential Access Method, a method for managing how a computer accesses records and files stored on a hard disk. While storing data sequentially, ISAM provides direct access to specific records through an index. This combination results in quick data access regardless of whether records are being accessed sequentially or randomly.

ISAM is a notion orthogonal to that of the relational database; it is not uncommon for relational databases to be implemented using an ISAM database library, the C-ISAM used by Informix being one example of this.

Most commonly, ISAM libraries are implemented atop some variation of the B-Tree data structure.

2.1. Main Memory Database Systems

These systems take advantage of the assumption (increasingly legitimate, with the cheap availability of massive amounts of RAM) that there is enough RAM to store the entire database in memory. That allows eliminating a whole host of sorts of analysis that disk-oriented or page-oriented databases have to do:

This does not have the forcible result of making the database vastly more fragile; if the updates are faithfully logged to disk, these systems can still maintain the ACID transaction requirements.

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