Re: enhance wraparound warnings

On Sat, Mar 21, 2026 at 4:16 AM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>
> On Mon, Mar 09, 2026 at 08:08:54PM +0800, wenhui qiu wrote:
> > Thank you for working on this. The path LGTM,...
>
> Thanks for looking. Committed.

Thanks for working on this feature! I have one comment.

ereport(WARNING,
(errmsg("database \"%s\" must be vacuumed within %u transactions",
oldest_datname,
xidWrapLimit - xid),
+ errdetail("Approximately %.2f%% of transaction IDs are available for use.",
+ (double) (xidWrapLimit - xid) / (MaxTransactionId / 2) * 100),
errhint("To avoid transaction ID assignment failures, execute a
database-wide VACUUM in that database.\n"
"You might also need to commit or roll back old prepared
transactions, or drop stale replication slots.")));

The new DETAIL messages for XID and MultiXactId wraparound warnings
seem misleading.

The percentage is calculated as (xidWrapLimit - xid) divided by half
of the ID space. This represents the remaining ID space before wraparound,
not the percentage of IDs that are still available for use. Since
PostgreSQL stops assigning new XIDs/MultiXactIds at the stop limit
before reaching the wraparound limit, the current wording could be
interpreted as overstating how many IDs remain usable.

So, how about changing the wording to match the calculation? For example:

Approximately XX.XX% of transaction ID space remains before wraparound.

and similarly for MultiXactIds.

Regards,

--
Fujii Masao