Wednesday, April 14, 2004, 1:16:54 AM, Andrew Piskorski wrote:
> as far as I can tell, it seems to be describing a system with
> the usual Oracle/PostgreSQL MVCC semantics, EXCEPT of course that
> Currie proposes that each Write transaction must take a lock on the
> database as a whole.

Well, i suppose from a sufficient distance they look alike,
but in practice MVCC and shadow paging are rather different.

In MVCC, each row typically has two hidden fields identifying the first
and last transaction ids for which the row is relevant.
The last transaction id is to skip rows that are deleted.
There are many variants of MVCC, but you get the idea.

Any reader (or writer) knows its own transaction id, and just
ignores rows that are no applicable.

A "vacuum" process is necessary to periodically reclaim space
taken by rows whose last transaction id is lower than any live

In shadow paging, the basic idea is that any reader or writer
gets a view onto the data based on reachability from "pointers"
in a particular root block. Pages that are reachable from any
live root block are never modified. A vacuum process is required 
to collect the space from blocks that are no longer reachable.
Updates to indexes must be treated in roughly the same way as
data pages, because they contain pointers to different data.

Shadow paging can be used for a table-based database, or
a persistent object store.
It certainly is much older than the HUT work; see for example Lorie 77,
"Physical Integrity in a Large Segmented Database."
It falls into the general class of logless transaction systems,
as opposed to the log-based approach that predominates in
current day non-academic database implementations.


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