Re: Incorrect result of bitmap heap scan.

On Mon, Dec 2, 2024 at 11:52 AM Andres Freund <andres@anarazel.de> wrote:
> I think the problematic scenario involves tuples that *nobody* can see. During
> the bitmap index scan we don't know that though.

Right, exactly.

FWIW, this same issue is why it is safe for nbtree to drop its pin
early during plain index scans, but not during index-only scans -- see
_bt_drop_lock_and_maybe_pin, and the nbtree/README section on making
concurrent TID recycling safe. Weirdly, nbtree is specifically aware
that it needs to *not* drop its pin in the context of index-only scans
(to make sure that VACUUM cannot do unsafe concurrent TID recycling)
-- even though an equivalent index scan would be able to drop its pin
like this.

The underlying reason why nbtree can discriminate like this is that it
"knows" that plain index scans will always visit the heap proper. If a
TID points to an LP_UNUSED item, then it is considered dead to the
scan (even though in general the heap page itself might be marked
all-visible). If some completely unrelated, newly inserted heap tuple
is found instead, then it cannot be visible to the plain index scan's
MVCC snapshot (has to be an MVCC snapshot for the leaf page pin to get
dropped like this).

-- 
Peter Geoghegan