Re: Improve description of XLOG_RUNNING_XACTS

On Sat, Oct 15, 2022 at 4:58 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> > I spent some time today reading this. As others said upthread, the
> > output can be more verbose if all the backends are running max
> > subtransactions or subtransactions overflow occurred in all the
> > backends.
> >
>
> As far as I can understand, this contains subtransactions only when
> they didn't overflow. The latest information provided by Sawada-San
> for similar records (XLOG_STANDBY_LOCK and XLOG_INVALIDATIONS) made me
> think that maybe we are just over-worried about the worst case.

Agreed. I see the below comment, which means when
xlrec->subxid_overflow is set to true, there will not be any
subtransaction ids logged in the WAL record.

 * Note that if any transaction has overflowed its cached subtransactions
 * then there is no real need include any subtransactions.
 */
RunningTransactions
GetRunningTransactionData(void)

If my above understanding is correct, having something like below does
no harm, like Masahiko-san's one of the initial patches, no? I'm also
fine with the way it is in the v3 patch.
if (xlrec->subxid_overflow)
{
   /*
    * Server doesn't include any subtransactions if any transaction has
    * overflowed its cached subtransactions.
    */
   Assert(xlrec->subxcnt == 0)
   appendStringInfoString(buf, "; subxid overflowed");
}
else if (xlrec->subxcnt > 0)
{
    appendStringInfo(buf, "; %d subxacts:", xlrec->subxcnt);
    for (i = 0; i < xlrec->subxcnt; i++)
        appendStringInfo(buf, " %u", xlrec->xids[xlrec->xcnt + i]);
}

The v3 patch posted upthread LGTM and I marked it as RfC. I'm just
reattaching the v3 patch posted upthread herewith so that the cfbot
can test the right patch - https://commitfest.postgresql.org/40/3779/.

> >
>  This can blow-up the output.
> >
>
> If we get some reports like that, then we can probably use Michael's
> idea of displaying additional information with a separate flag.

Agreed.

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com