Re: [COMMITTERS] pgsql: Reduce spurious Hot Standby conflicts from never-visible records

(Moving to pgsql-hackers)

On 10.12.2010 20:21, Tom Lane wrote:
> Simon Riggs<simon(at)2ndQuadrant(dot)com> writes:
>> Reduce spurious Hot Standby conflicts from never-visible records.
>> Hot Standby conflicts only with tuples that were visible at
>> some point. So ignore tuples from aborted transactions or for
>> tuples updated/deleted during the inserting transaction when
>> generating the conflict transaction ids.
>
>> Following detailed analysis and test case by Noah Misch.
>> Original report covered btree delete records, correctly observed
>> by Heikki Linnakangas that this applies to other cases also.
>> Fix covers all sources of cleanup records via common code.
>> Includes additional fix compared to commit on HEAD
>
> ISTM HeapTupleHeaderAdvanceLatestRemovedXid is still pretty broken,
> in that it's examining xmax without having checked that xmax is (a)
> valid or (b) a lock rather than a deletion xmax.

In current use, it's only called for tuples that are known to be dead,
so either xmax is a valid deletion, or xmin didn't commit in which case
the function doesn't use xmax for anything. So I think it actually works
as it is.

I agree it doesn't look right, though. At the very least it needs
comments explaining that, but preferably it should do something sane
when faced with a tuple that's not dead after all. Perhaps throw an
error (though that would be bad during recovery), or an Assert, or just
refrain from advancing latestRemovedXid (or advance it, that would be
the conservative stance given the current use).

Also, I'm not totally convinced it's correct when xmin > xmax, despite
Simon's follow-up commit to fix that. Shouldn't it advance
latestRemovedXid to xmin in that case? Or maybe it's ok as it is because
we know that xmax committed after xmin. The impression I get from the
comment above the function now is that it advances latestRemovedXid to
the highest XID present in the tuple, but that's not what it does in the
xmin > xmax case. That comment needs clarification.

While we're at it, perhaps it would be better to move this function to
tqual.c. And I feel that a more natural interface would be something like:

TransactionId
HeapTupleHeaderGetLatestRemovedXid(HeapTupleHeader tuple);

IOW, instead bumping up the passed-in latestRemovedXid value, return the
highest XID on the tuple (if it was dead).

PS. it would be good to set hint bits in that function like in
HeapTupleSatisfies* functions.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com