After a talk with my local mainframe guru, here is some more information about the concepts of ISAM and VSAM databases.

ISAM databases are little more than flat (row oriented) files together with an index (file) that gives the start offsets of the differen rows in the data file. Reading and sorting is only possible in one order, the order determined by the index.

VSAM databases have more than one index and also have the concept of "keys" for the data. You can traverse the data along several axes, as determined by the keys, so you don't have to scan the whole database on each query.

Both databases have no intrinsic support for hierarchies or joined queries.

The "successor" to VSAM and ISAM is DL1, a somewhat hierarchical storage of datasets which was famed for its raw speed. After that came DB2 and SQL as the mediator between the programmer and the database (and thus, the interest in different database concepts faded as all DBs are accessible via SQL).

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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