Re: Partial vacuum versus pg_class.reltuples

I wrote:
> Another interesting question is why successive vacuums aren't causing
> the index reltuples counts to go to zero. Shouldn't a partial vacuum
> result in *all* pages of the relation being marked as not needing to
> be examined by the next vacuum?

I figured out the reason for that: the first 32 pages of the table are
always scanned, even if the whole thing is frozen, because of the
SKIP_PAGES_THRESHOLD logic. We could change that behavior by
initializing all_visible_streak to SKIP_PAGES_THRESHOLD instead of zero.
But if we did so then having even just page zero be skippable would mean
that we clear scanned_all and thus fail to update reltuples, which is
probably not a good thing.

regards, tom lane