The following review has been posted through the commitfest application:
make installcheck-world:  tested, failed
Implements feature:       tested, failed
Spec compliant:           tested, failed
Documentation:            tested, failed

Hi! I've looked into the patch.

There is not so much of the code. The code itself is pretty clear and well 
documented. I've found just one small typo "transactionn" in 
HeapTupleSatisfiesNonVacuumable() comments.

Chosen Snapshot type is not a solution to the author's problem at hand. Though 
implemented SnapshotNonVacuumable is closer to rational choice  than currently 
used SnapshotDirty for range sketching. As far as I can see this is what is 
get_actual_variable_range() is used for, despite word "actual" in it's name. 
The only place where get_actual_variable_range() is used for actual range is 
preprocessed-out in get_variable_range():
         * XXX It's very tempting to try to use the actual column min and max, 
         * we can get them relatively-cheaply with an index probe.  However, 
         * this function is called many times during join planning, that could
         * have unpleasant effects on planning speed.  Need more investigation
         * before enabling this.
#ifdef NOT_USED
        if (get_actual_variable_range(root, vardata, sortop, min, max))
                return true;

I think it would be good if we could have something like "give me actual 
values, but only if it is on first index page", like conditional locks. But I 
think this patch is a step to right direction.

Best regards, Andrey Borodin.
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