On Thu, Oct 12, 2017 at 4:38 PM, Alexander Korotkov
<a.korot...@postgrespro.ru> wrote:
> It's probably that we imply different meaning to "MVCC implementation".
> While writing "MVCC implementation" I meant that, for instance, alternative
> storage
> may implement UNDO chains to store versions of same row.  Correspondingly,
> it may not have any analogue of our HOT.

Yes, the zheap project on which EnterpriseDB is working has precisely
this characteristic.

> However I imply that alternative storage would share our "MVCC model".  So,
> it
> should share our transactional model including transactions,
> subtransactions, snapshots etc.
> Therefore, if alternative storage is transactional, then in particular it
> should be able to fetch tuple with
> given TID according to given snapshot.  However, how it's implemented
> internally is
> a black box for us.  Thus, we don't insist that tuple should have different
> TID after update;
> we don't insist there is any analogue of HOT; we don't insist alternative
> storage needs vacuum
> (or if even it needs vacuum, it might be performed in completely different
> way) and so on.

Fully agreed.

> During conversations with you at PGCon and other conferences I had
> impression
> that you share this view on pluggable storages and MVCC.  Probably, we just
> express
> this view in different words.  Or alternatively I might understand you
> terribly wrong.

No, it sounds like we are on the same page.  I'm only hoping that we
don't end with a bunch of storage engines that each use a different
XID space or something icky like that.  I don't think the API should
try to cater to that sort of development.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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