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From "Mike Kienenberger" <mkien...@gmail.com>
Subject Re: temporal database generation
Date Fri, 25 Aug 2006 15:30:05 GMT
In my opinion, (and maybe I'm short-sighted), this is really an
application-level thing.

If I were going to implement it to be universal, I'd subclass
DataContext (easy to do with a DataContextFactory), have a timestamp
property on it, and automatically update what happens when a query is
performed (append date between expressions to each table -- this is
probably the hard part) and committed (automatically update the date
fields).


On 8/25/06, Tomi NA <hefest@gmail.com> wrote:
> I've been tasked with designing a system, including it's database to
> capture data which changes through time but shouldn't overwrite
> existing values. I've also ran into other people describing these
> kinds of databases in their work, referring to them as "temporal
> databases".
> Here's why I'm writing to the cayenne list about it. I've been
> thinking...say you started with a regular database model (a cayenne
> model or an existing db model) like you would on any project. The
> modeler could conceivably generate the whole thing: a pair of
> "valid_from" - "valid_to" fields in every table, code which would
> transform an update command into a relevant update/insert command pair
> and finally, code which would allow transparent access to the most
> current information stored in the database.
> The generated API would be almost identical to the one cayenne
> generates now for a "plain" database, the only exception being the
> possibility to define a timestamp (maybe at the context level) which
> would allow you to do the same (SELECT) queries, but effectively
> moving back in time to the desired moment.
>
> Concievably, you could do something like this:
> DataContext dc = ...;
> dc.setPointInTime(beginningOfTheYear);
> // prints out the number of employees on 2006-01-01
> System.out.println(dc.performQuery(new SelectQuery(Employee.class)).size());
> dc.setPointInTime(now);
> // prints out the number of employees on 2006-08-25
> System.out.println(dc.performQuery(new SelectQuery(Employee.class)).size());
>
> Obviously, a database like this would grow possibly very fast so it
> might not be a suitable design where space is a major concern, but I
> can think of a lot of applications where this would be a fantastic
> thing to have out of the box.
>
> I'd appreciate anyone with an opinion on this to comment.
>
> TIA,
> t.n.a.
>

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