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From "Dibyendu Majumdar" <dibye...@mazumdar.demon.co.uk>
Subject Re: Derby architecture/design documents
Date Mon, 31 Jan 2005 19:14:08 GMT
Mike,

A couple of questions for you.

While documenting the page formats, I noticed that Derby does not persist
the LogInstant (LSN) associated with a page. In a classic ARIES
implementation, each page is supposed to be tagged with the LSN of the log
record that last updated it, so that during recovery redo, the LSN can be
used to determine if a particular log record has already been applied to the
page.

Derby appears to use PageVersion for this purpose, which is persisted and is
also present in log records. Any reason why PageVersion is used and not
LogInstant?

Another thing that has puzzled me is the use of RecordIds. I would have
assumed that the purpose of the slot table in a page is provide a stable
TupleId (as described in the classic TPCT book by Jim Gray and Andreas
Reuter), so that there is freedom to move records around in the page, and
still have a stable TupleId that does not change over time. Also, the
slot-no allows a particular record to be located efficiently. Given that a
page in Derby has a slot table, why is the slot-no not used as part of the
RecordId? Using a separate id is obviously inefficient, as it must be
searched for.

Thanks and Regards

Dibyendu





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