Re: [PROPOSAL] Use SnapshotAny in get_actual_variable_range

On Thu, May 4, 2017 at 9:42 PM, Dmitriy Sarafannikov
<dsarafannikov(at)yandex(dot)ru> wrote:
>
>> Maybe we need another type of snapshot that would accept any
>> non-vacuumable tuple. I really don't want SnapshotAny semantics here,
>> but a tuple that was live more recently than the xmin horizon seems
>> like it's acceptable enough. HeapTupleSatisfiesVacuum already
>> implements the right behavior, but we don't have a Snapshot-style
>> interface for it.
>
>
> I have tried to implement this new type of snapshot that accepts any
> non-vacuumable tuples.
> We have tried this patch in our load environment. And it has smoothed out
> and reduced magnitude of the cpu usage peaks.
> But this snapshot does not solve the problem completely.
>
> Patch is attached.

1.
+#define InitNonVacuumableSnapshot(snapshotdata) \
+ do { \
+ (snapshotdata).satisfies = HeapTupleSatisfiesNonVacuumable; \
+ (snapshotdata).xmin = RecentGlobalDataXmin; \
+ } while(0)
+

Can you explain and add comments why you think RecentGlobalDataXmin is
the right to use it here? As of now, it seems to be only used for
pruning non-catalog tables.

2.
+bool
+HeapTupleSatisfiesNonVacuumable(HeapTuple htup, Snapshot snapshot,
+ Buffer buffer)
+{
+ return HeapTupleSatisfiesVacuum(htup, snapshot->xmin, buffer)
+ != HEAPTUPLE_DEAD;
+}
+

Add comments on top of this function and for the sake of consistency
update the file header as well (Summary of visibility functions:)

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