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(at)gmail(dot)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 and need to perform maintenance activity.
> > Another one is the same data causes an update conflict because two different transactions can update the same version of 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

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Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

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