On Thu, Oct 12, 2017 at 8:00 PM, Haribabu Kommi
<kommi.harib...@gmail.com> 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.

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

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