Re: Stored fields ordered left to right
Date: Sat, 27 Dec 2003 09:40:41 -0600
Message-ID: <bsk95t$sbh$1_at_news.netins.net>
"Jerry Gitomer" <jgitomer_at_erols.com> wrote in message
news:3fed1185$0$4736$61fed72c_at_news.rcn.com...
> Dawn M. Wolthuis wrote:
> > "Joe "Nuke Me Xemu" Foster" <joe_at_bftsi0.UUCP> wrote in message
> > news:1072496216.517226_at_news-1.nethere.net...
> >
> >>"Dawn M. Wolthuis" <dwolt_at_tincat-group.com> wrote in message
> >
> > <news:bsira5$jlt$1_at_news.netins.net>...
> >
> <snip>
> >> >>>certainly not relations, and the system is certainly not relational)."
> >>>Date writes "...MVS fields are ordered left to right (and so MVS files
> >
> > are
> >
> >>>
>
> <big snip>
>
> Allow me to play the role of the fool jumping in where angels
> fear to tread.....
>
> Two points which may clarify RDBMS implementation (as opposed to
> theory).
>
> 1. The relationships are imposed externally to the data in the
> form of indexes and/or foreign keys. The data itself is
> unordered. If I remember correctly Codd specifically stated
> that the data was not ordered. From an implementation point of
> view (circa 1970 when the largest mainframes weren't much faster
> than my Palm IIIc) this allowed signifiantly better performance
> when adding rows that should logically be anyplace other than
> the end of the table.
>
> 2. Within a table row the physical order of the columns as
> stored on disk need not conform to the logical order of the
> columns as specified in the CREATE TABLE statement. Again
> looking at the mainframe computers of 1970 when all but the very
> largest had less than 16MB of RAM, the highest capacity disk
> drives only had 33MB of storage and there were arcane rules
> about floats and integers starting on word boundaries while
> short integers and strings could start on any byte in a word it
> became desirable to store all of the floats and integers at the
> beginning of the physical disk record and the shorts and strings
> after them in order not to waste any space in either memory or
> on disk.
>
> HTH
Yes, this is most helpful. This is PRECISELY my understanding -- that deciding to remove the ordering from relational tuples is an implementation issue and not about the logical theory of relations.
I work with relations that are mathematical relations and are therefore ordered tuples. The model behind XML documents is also one of ordered tuples. So, if you hear of folks who might sometimes spout that their database model is "more relational" than RDBMS's it sometimes is due to this particular issue.
Based on this, it sounds like a response to Date that says that mathematical relations are ORDERED and not unordered tuples so that this particular point is irrelevant (and, in fact, wrong) would be an accurate response, right?
Thanks a bunch! --dawn Received on Sat Dec 27 2003 - 16:40:41 CET