A related point comes to mind, consider the single
"tablespace" file
example: It may have no fragmentation from a *file*
perspective, but
supposing you have several busy relations being inserted or updated
then pages (or groups of pages) for these could be "fragmented"
throughout the tablespace file.
+1 ... what was said upthread sounds to me like those other databases
are just hiding the fragmentation issue within their
huge-files-you-can't-see-into. I would very much like to see
some proof of performance problems before we worry about this.
Yes, they definitly do. That's why for exapmle SQLServer provides the
commands "DBCC CHECKCONTIG" and "DBCC INDEXDEFRAG". Along with comments
like this:

"DBCC SHOWCONTIG determines whether the table is heavily fragmented.
Table fragmentation occurs through the process of data modifications
(INSERT, UPDATE, and DELETE statements) made against the table. Because
these modifications are not ordinarily distributed equally among the
rows of the table, the fullness of each page can vary over time. For
queries that scan part or all of a table, such table fragmentation can
cause additional page reads. This hinders parallel scanning of data."


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