Tom Lane wrote:
> "Kevin Grittner" writes:
>> By not visiting the heap page for tuples, index-only scans fail to
>> acquire all of the necessary predicate locks for correct behavior
>> at the serializable transaction isolation level. The tag for the
>> tuple-level predicate locks includes the xmin, to avoid possible
>> problems with tid re-use. (This was not covered in initial
>> pre-release versions of SSI, and testing actually hit the
>> problem.) When an "index-only" scan does need to look at the heap
>> because the visibility map doesn't indicate that the tuple is
>> visible to all transactions, the tuple-level predicate lock is
>> acquired. The best we can do without visiting the heap is a page
>> level lock on the heap page, so that is what the attached patch
>> does.
>
>> If there are no objections, I will apply to HEAD and 9.2.
>
> This isn't right in detail: there are paths through the loop where
> "tuple" is not NULL at the beginning of the next iteration
> (specifically, consider failure of a lossy-qual recheck). I think
> that only results in wasted work, but it's still not operating as
> intended. I'd suggest moving the declaration/initialization of the
> "tuple" variable to inside the while loop, since there's no desire
> for its value to carry across loops.
You're right. It looks to me like moving the declaration (and
initialization) to more local scape (just inside the loop) fixes it.
New version attached. Will apply if no further problems are found.
-Kevin