Re: MVCC snapshot timing

On Tue, Nov 12, 2013 at 05:35:23PM -0800, David Johnston wrote:
> Bruce Momjian wrote
> > On Tue, Nov 12, 2013 at 03:36:01PM -0800, David Johnston wrote:
> >> Bruce Momjian wrote
> >> > We just want to get across the MVCC concept in the intro --- we cover
> >> > the snapshots later in the document.
> >>
> >> I just think we're being too vague here; and we are covering them in the
> >> intro with the use of "some point in the past".
> >>
> >> IMO, the main point regarding MVCC is that every change in the system
> >> creates a new record and causes a prior record to be invalidated at a
> >> point-in-time.  The combination of these two things increases concurrency
> >> since you can create new records while people are still using the old
> >> ones.
> >> One consequence, though, is that it is necessary for the user to decide
> >> at
> >> what point in the timeline they want to view the database.
> >>
> >> Does this sound right?
> >
> > I still do not see how this fits appropriately in the introduction.
>
> The concept or the actual wording?
>
> The intended question was whether my understanding (and simplification) of
> the concept is correct.
>
> My specific wording is incoherent mostly because it really belongs to a
> larger corpus that currently exists only in my head.

Oh, OK, it sounds fine.  The user really doesn't choose what timeline to
see --- rather, it is the current xid at the time they take their
snapshot and other running xids that controls that.  You can control
your transaction isolation level, but that only controls how often you
take snapshots.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +