Re: xact_rollback spikes when logical walsender exits

Fujii-san, coming back to your questions in this thread, I should have
addressed
them before sending v2 -- apologies for jumping past them.

On Mon, Apr 20, 2026 at 9:35 AM Fujii Masao <masao.fujii@gmail.com> wrote:

> Thanks for the report and patch!
>
> How to implement a solution depends on what xact_rollback in
> pg_stat_database
> is intended to mean. So at first we should consider which rollbacks should
> it count? The documentation does not currently give an explicit definition.
>
> At present, xact_rollback appears to count all rollbacks, explicit or
> implicit,
> by any process connected to the database, including regular backends,
> autovacuum workers, and logical walsenders. If that is the intended
> definition,
> then rollbacks implicitly performed by logical walsenders during logical
> replication should also be counted. Of course, even if we keep that
> definition,
> the sudden increase in xact_rollback might still be a problem, so we might
> need to call pgstat_report_stat() immediately after pgstat_flush_io() in
> walsender, so the counters continue to be updated periodically during
> logical replication.
>
> On the other hand, your patch seems to assume a different definition: that
> xact_rollback should count all explicit and implicit rollbacks, except
> those
> performed by logical walsenders during logical replication. That would be
> one possible approach, although it seems a bit odd to exclude only one
> subset
> of rollbacks.
>
> A third option would be to define xact_rollback more narrowly, counting
> only
> rollbacks by regular backends, and excluding rollbacks by processes such as
> autovacuum or walsender. At least in my view, xact_commit and xact_rollback
> in pg_stat_database are typically used by DBAs to check whether
> client transactions are committing or rolling back as expected. From
> that perspective, it seems intuitive for xact_rollback to count only
> rollbacks
> by regular backends. But others may reasonably see it differently.
>

On the definitional question: I'd argue the walsender case isn't a
"subset of rollbacks being excluded". The transactions being decoded
actually committed; the abort in ReorderBufferProcessTXN() is
internal cleanup of the catalog-snapshot transaction, not a
transaction outcome. Notably, xact_commit is not incremented for
these, so on any publisher the commit/rollback pair becomes
internally inconsistent in proportion to decoded throughput. So
counting these as rollbacks seems like a miscount under any of the
three definitions.

On option 1 (periodic pgstat_report_stat() in walsender): that would
trade the exit spike for a steady inflated rollback rate, which
seems worse for the metric's usefulness. Periodic flushing may still
be worth doing independently for other stats; happy to send that as
a separate patch.

On option 3: I agree this matches how DBAs actually use these
counters, and I'd support it. Since it changes observable behavior
for autovacuum etc., it seems like master-only material, together
with explicitly documenting the definition.

Concretely, I propose splitting:

1) 0001 (v3, attached): narrow fix for the walsender miscount,
intended as a back-patchable bug fix -- this is what's paging people
in production today. Rebased on current master; the TAP test now
also verifies that xact_commit does not change as a function of
decoded transactions (walsender shutdown has a small fixed
bookkeeping delta, handled via a control run).

2) 0002 (attached, draft): master-only implementation of option 3 --
count xact_commit/xact_rollback only for regular client backends --
plus a doc change defining both columns explicitly. It also removes
the now-redundant parallel argument from AtEOXact_PgStat() and
AtEOXact_PgStat_Database(), since parallel workers are background
workers and are excluded by the backend-type check. 0002 has no test
yet; if the definition is agreed on, I'll extend the new TAP test to
cover the autovacuum/walsender exclusion.

One consequence of option 3 worth stating explicitly: logical
replication apply workers are background workers, so transactions
they replay on a subscriber would no longer be counted in the
subscriber's xact_commit. That's arguably correct under the "client
transactions" definition, but it does change what subscriber-side
monitoring sees, so I wanted to flag it rather than have it
discovered later.

Added to the July commitfest: https://commitfest.postgresql.org/patch/6992/

Looking forward to your thoughts.

Nik