Multivalue datatypes

What a lot of people like repeating groups...

By David Norfolk • Get more from this author

Posted in Developer, 20th July 2006 15:07 GMT

Editors blog Mark Whitehorn's recent article on multivalue data types in Access (here) sure attracted some comments.

No, multivalue fields (non-atomic fields or repeating groups as I think I remember them) are not new. I didn't like them when I was an IMS DBA back in the eighties - and I met most of the arguments in favour of them back then. But, whatever Oracle does, they aren't part of a "proper" relational database.

To set the matter straight and to avoid unnecessary email traffic:

Yes, we have heard of Dick Pick;
yes, we do realise that multivalued datatypes can sometimes be used safely if you're careful; or your DBMS is (a bit like GoTos, and with similar caveats);
yes, we do realise that databases like U2, ADABAS etc have their uses and that Cache is actually a very interesting database, which is much more than just a modern implementation of Pick or Mumps...

However, I still think that Relational Theory (as in Codd's ACM paper) is a "good thing" and that it simplifies the delivery of reliable, resilient systems in the "real world"; and that XML columns, fields containing sets, Blobs and similar modern non-relational additions, while expedient, re-introduce some of the issues which lead to IMS (which I know very well), say, being replaced with DB/2 or other RDBMSs - in general.

And, FWIW, I do also realise that IMS, as well as IDMS, CA Datacom and other legacy databases are still in active use, because the benefits from keeping them (and, as well as the simple replacement and regression-testing costs, there are sometimes resource/performance benefits from using unnormalised data for specific access opaths; there are also consequences) outweigh the possible benefits from replacing them.

A longer article justifying these positions in detail will take a little time to prepare - but watch this space...®

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