On Tue, Aug 26, 2014 at 8:32 PM, Jeff Davis <pgsql@j-davis.com> wrote:
> We are mixing two kinds of data: user data and visibility information.
> Each is changed under different circumstances and has different
> characteristics, and I'm beginning to think they shouldn't be mixed at
> all.
>
> What if we just devised a structure specially designed to hold
> visibility information, put all of the visibility information there, and
> data pages would only change where there is a real, user-initiated
> I/U/D. Vacuum could still clear out dead tuples from the data area, but
> it would do the rest of its work on the visibility structure. It could
> even be a clever structure that could compress away large static areas
> until they become active again.
Well fundamentally the reason the visibility information is with the
user data is so that we don't need to do two i/os to access the data.
The whole point of hint bits is to guarantee that after some amount of
time you can read data directly out of the heap page and use it
without doing any additional I/O.
--
greg