Re: Doubt in mvcc

On Mon, Jul 13, 2020 at 10:41:28AM +0200, Francisco Olarte wrote:
> Rama:
> 
> On Mon, Jul 13, 2020 at 9:52 AM Rama Krishnan <raghuldrag@gmail.com> wrote:
> > I m preparing for interview one of the recruiter asked me mvcc drawbacks as i told due to mvcc it use more space
andneed to perform maintenance activity.
 
> > Another one is the same data causes an update conflict because two different transactions can update the same
versionof the row.
 
> >  he told its wrong, kindly tell me will you please tell me its correct or wrong?
> 
> I'm not sure I understand your question too well, you may want to
> refresh/expand.
> 
> One interpretation is, on a pure MVCC contest, two transactions, say 5
> and 6, could try to update a tuple valid for [1,) and end up
> generating two new tuples, [5,), [6,) and closing the original at
> either [1,5) or [1,6) .
> 
> That's why MVCC is just a piece, locking is other. On a MVCC the
> tuples are locked while a transaction manipulates them. Other
> transactions may read them, which is why readers do not block writers,
> but two updates on the same tuple serialize.

You might want to look at this:

    https://momjian.us/main/presentations/internals.html#mvcc

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

  The usefulness of a cup is in its emptiness, Bruce Lee