Re: cannot freeze committed xmax

On 28.10.2020 18:25, Mark Dilger wrote:
>
>> On Oct 28, 2020, at 6:44 AM, Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru> wrote:
>>
>> Looks like there is no assumption that xmax should be set to InvalidTransactionId when HEAP_XMAX_INVALID bit is set.
>> And I didn't find any check preventing cutoff_xid to be greater than XID of some transaction which was aborted long time ago.
> Nothing in your example suggests that cutoff_xid is wrong, so I'd assume that part is probably working ok.
>
> Your data shows that HEAP_XMAX_INVALID and HEAP_UPDATED flags are both set. That should only happen if the updating transaction aborted. But the query of clog is saying that it committed. Something is wrong with that. How did the hint bits get set to HEAP_XMAX_INVALID if the transaction did commit. Either some process is setting that hint bit when it shouldn't, or your clog is corrupted and returning a bogus answer about the xmax having been committed. Either way, you've got corruption.
>
> Your question "preventing cutoff_xid to be greater than XID of some transaction which was aborted long time ago" seems to be ignoring that TransactionIdDidCommit(xid) is returning true, suggesting the transaction did not abort.
>
> —
> Mark Dilger
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
Yes, I forgot to say that transaction is treated as committed
(txid_status() returns "committed").
Also database was previously upgraded from 11.5 to 11.7
Certainly the hypothesis of CLOG corruption explains everything.
I wonder if there can be some other scenario (upgrade, multixacts,
previous freeze attempt...) which can cause such combination of flags?
I have inspected all cases where HEAP_XMAX_INVALID is set, but have not
found any one which can explain it.

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company