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Re: Pre-relational, post-relational, 1968 CODASYL "Survey of Data Base Systems"

From: Ken North <knorth2_at_deletethis.yahoo.com>
Date: Thu, 29 Apr 2004 14:55:37 -0700
Message-ID: <c6rth2$ihn$1@ngspool-d02.news.aol.com>


> one of the big arguments that i remember from the pre-relational dbms
> people in stl ... was the physical disk space requirements. The
> earlier databases had physical pointers ... which relational replaced
> with indexes. The indexes tended to double the physical space
> requirements (as well as increasing overhead) vis-a-vis the databases
> with direct physical pointers. The indexes, however, reduced the
> manual maint. involved in maintaining the physical pointers. The issue
> then was somewhat the disk space & processing overhead vis-a-vis the
> manual maint overhead.

A CODASYL (network model), pre-relational database contains sets of records. Each set has an owner record and 1-n member records. The organization is similar in concept to the persistent representation of a doubly-linked list. There are pointers to the next and prior members of a set, but the CODASYL model also has a pointer to the owner of the set.

The database designer or database administrator (DBA) define the set relationships in a schema that describes names, types, lengths, owner and member relationships. Using a network model database manager puts a premium on detailed design before you load data. There's no easy solution equivalent to SQL's ALTER TABLE. Making changes to the structure of a CODASYL database is non-trivial. If your database design uses the wrong length or type for a field (attribute), or omits one, you have to re-compile the schema and re-load the database to recreate the pointers. Received on Thu Apr 29 2004 - 16:55:37 CDT

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