Re: Incorrect result of bitmap heap scan.

Hi,

On 2024-12-02 11:43:42 -0500, Tom Lane wrote:
> Peter Geoghegan <pg@bowt.ie> writes:
> > This theory seems very believable.
> 
> I'm not convinced.  I think there are two assumptions underlying
> bitmap scan:
> 
> 1. Regardless of index contents, it's not okay for vacuum to remove
> tuples that an open transaction could potentially see.  So the heap
> tuple will be there if we look, unless it was already dead (in which
> case it could have been replaced, so we have to check visibility of
> whatever we find).

I think the problematic scenario involves tuples that *nobody* can see. During
the bitmap index scan we don't know that though. Thus the tid gets inserted
into the bitmap. Then, before we visit the heap, a concurrent vacuum removes
the tuple from the indexes and then the heap and marks the page as
all-visible, as the deleted row version has been removed.  Then, during the
bitmap heap scan, we do a VM lookup and see the page being all-visible and
thus assume there's a visible tuple pointed to by the tid.

No snapshot semantics protect against this, as the tuple is *not* visible to
anyone.

Greetings,

Andres Freund