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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Looking for Indexed Sequential File Package
Date: 1997/07/20
Message-ID: <dewar.869405494@merv>#1/1
X-Deja-AN: 257841561
References: <33CCA95E.167EB0E7@avions.aerospatiale.fr>
 <5qlskj$3ka@top.mitre.org> <1997Jul18.144059.1@eisner>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-07-20T00:00:00+00:00
List-Id: <comp.lang.ada>


Michael says

<<> Indexed sequential files are not always called that anymore. An indexed
> sequential file is a data structure with two components: an indexing
> container which keeps a list of keys in sorted order, and a random
> access container (falsely called sequential in the title Indexed Sequential).
>>


An indexed sequential file is one which is organized so that sequential
acesses are very rapid once an index location has been looked up. Typically
you can access sequentially on the basis of more than one index. So this
means that hash schemes are totally useless for example.

The structures used for these files on modern operating systems are
extremely sophisticated, involving interaction with the virtual memory
system, and also elaborate methods of key compression (the average size
of a key in Realia COBOL for example is about 12 bits -- this is independent
of the actual key length).