Re: [HACKERS] Pluggable storage

On Fri, Oct 13, 2017 at 1:58 PM, Haribabu Kommi
 wrote:
>
> On Fri, Oct 13, 2017 at 11:55 AM, Robert Haas  wrote:
>>
>> On Thu, Oct 12, 2017 at 8:00 PM, Haribabu Kommi
>>  wrote:
>> > 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.
>>
>> That seems like a strange choice of API.  I think it should more
>> integrated with the scan logic.  For example, if I'm doing an index
>> scan, and I get a TID, then I should be able to just say "here's a
>> TID, give me any tuples associated with that TID that are visible to
>> the scan snapshot".  Then for the current heap it will do
>> heap_hot_search_buffer, and for zheap it will walk the undo chain and
>> return the relevant tuple from the chain.
>
>
> OK, Understood.
> I will check along these lines and come up with storage API's.
>

I think what we need here is a way to register satisfies function
(SnapshotSatisfiesFunc) in SnapshotData for different storage engines.
That is the core API to decide visibility with respect to different
storage engines.


-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com


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