Re: Stored fields ordered left to right

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

>>>Date writes "...MVS fields are ordered left to right (and so MVS files

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 Received on Sat Dec 27 2003 - 05:58:50 CET