Re: Pluggable storage

On Fri, Oct 13, 2017 at 8:23 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Thu, Oct 12, 2017 at 4:38 PM, Alexander Korotkov
> <a(dot)korotkov(at)postgrespro(dot)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.

Currently I added a snapshot_satisfies API to find out whether the tuple
satisfies the visibility or not with different types of visibility
routines. I feel these
are some how enough to develop a different storage methods like UNDO.
The storage methods can decide internally how to provide the visibility.

+ amroutine->snapshot_satisfies[MVCC_VISIBILITY] = HeapTupleSatisfiesMVCC;
+ amroutine->snapshot_satisfies[SELF_VISIBILITY] = HeapTupleSatisfiesSelf;
+ amroutine->snapshot_satisfies[ANY_VISIBILITY] = HeapTupleSatisfiesAny;
+ amroutine->snapshot_satisfies[TOAST_VISIBILITY] = HeapTupleSatisfiesToast;
+ amroutine->snapshot_satisfies[DIRTY_VISIBILITY] = HeapTupleSatisfiesDirty;
+ amroutine->snapshot_satisfies[HISTORIC_MVCC_VISIBILITY] =
HeapTupleSatisfiesHistoricMVCC;
+ amroutine->snapshot_satisfies[NON_VACUUMABLE_VISIBILTY] =
HeapTupleSatisfiesNonVacuumable;
+
+ amroutine->snapshot_satisfiesUpdate = HeapTupleSatisfiesUpdate;
+ amroutine->snapshot_satisfiesVacuum = HeapTupleSatisfiesVacuum;

Currently no changes are carried out in snapshot logic as that is kept
seperate
from storage API.

Regards,
Hari Babu
Fujitsu Australia