Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On Tue, Oct 3, 2017 at 8:43 PM, Wong, Yi Wen <yiwong(at)amazon(dot)com> wrote:
> My interpretation of README.HOT is the check is just to ensure the chain is continuous; in which case the condition should be:
>
>> if (TransactionIdIsValid(priorXmax) &&
>> !TransactionIdEquals(priorXmax, HeapTupleHeaderGetRawXmin(htup)))
>> break;
>
> So the difference is GetRawXmin vs GetXmin, because otherwise we get the FreezeId instead of the Xmin when the transaction happened

I was thinking along similar lines.

> The interesting consequence of changing that is the prune seems to get the entire chain altogether with Dan's repro... I've run it a couple of times and have consistently gotten the following page
>
> lp | t_ctid | lp_off | lp_flags | t_infomask | t_infomask2
> ----+--------+--------+----------+------------+-------------
> 1 | (0,1) | 8152 | 1 | 2818 | 3
> 2 | | 7 | 2 | |
> 3 | | 0 | 0 | |
> 4 | | 0 | 0 | |
> 5 | | 0 | 0 | |
> 6 | | 0 | 0 | |
> 7 | (0,7) | 8112 | 1 | 11010 | 32771
> (7 rows)

That's also what I see. This is a good thing, I think; that's how we
ought to prune.

--
Peter Geoghegan