Re: UNDO and in-place update

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> [ Let's invent Oracle-style UNDO logs ]

I dunno. I remember being told years ago, by an ex-Oracle engineer,
that he thought our approach was better. I don't recall all the details
of the conversation but I think his key point was basically this:

> - Reading a page that has been recently modified gets significantly
> more expensive; it is necessary to read the associated UNDO entries
> and do a bunch of calculation that is significantly more complex than
> what is required today.

Oracle spends a lot of time on this, and it's really cache-inefficient
because the data is spread all over. This was what this guy felt in
circa 2001; I'd have to think that the cache unfriendliness problem is
much worse for modern hardware.

Which is not to say that it's not worth experimenting with. But what you
describe is going to be a huge amount of work even to get to the point
where we could try to measure the actual costs and benefits :-(

Heikki's been fooling with some ideas that I think have more promise.
I wish he'd get to the point of presenting them publicly rather than
just over beers at conferences.

regards, tom lane