Re: Proposal: Conflict log history table for Logical Replication

On Mon, Oct 12, 2026 at 11:31 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Mon, Oct 12, 2026 at 6:04 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Fri, Oct 9, 2026 at 1:43 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > Another concern on v79 is:
> > >
> > > I noticed that one CLT row contains three timestamp representations
> > > with three different behaviors:
> > >
> > > 1) replica_identity is frozen in the apply worker’s DateStyle.
> > > Session's Datestyle has no impact on it.
> > > 2) remote_commit_ts is represented in the reader’s (session's) DateStyle, and
> > > 3) local_conflicts.commit_ts is always ISO.
> > >
> > > This may cause some conufsion. Please see the results below. I can
> > > share the testcase if needed:
> > >
> > > Conflicts:
> > >
> > > postgres=# select conflict_type, remote_commit_ts, replica_identity,
> > > local_conflicts from pg_conflict.pg_conflict_log_16390;
> > > conflict_type | remote_commit_ts |
> > > replica_identity |
> > > local_conflicts
> > > -----------------------+--------------------------------+-----------------------------------+------------------------------------------------------------------------------------------
> > > delete_origin_differs | 09/10/2026 14:15:29.055423 IST |
> > > {"k":"2025-12-25 10:00:00+05:30"} |
> > > {"{\"xid\":\"672\",\"commit_ts\":\"2026-10-09T14:15:22.033391+05:30\",\"origin\":null}"}
> > > delete_origin_differs | 09/10/2026 14:17:12.929024 IST |
> > > {"k":"02/01/2025 08:00:00 IST"} |
> > > {"{\"xid\":\"678\",\"commit_ts\":\"2026-10-09T14:17:09.326423+05:30\",\"origin\":null}"}
> > > (2 rows)
> > >
> > >
> > > 1) First conflict row was inserted with default Datastyle of DB and
> > > the apply worker which is:
> > > postgres=# show DateStyle;
> > > DateStyle
> > > -----------
> > > ISO, DMY
> > >
> > > 2) Before inserting second conflict, below was done.
> > >
> > > ALTER DATABASE postgres SET DateStyle = 'SQL, DMY';
> > > ALTER SUBSCRIPTION sub1 DISABLE; ALTER SUBSCRIPTION sub1 ENABLE;
> > >
> > > (So Above conflict data is read when session's format was also 'SQL,
> > > DMY', thus we see remote_commit_ts in SQL format)
> > > ----
> > >
> > > We can see the differnce in 'replica_identity' values in 2 rows, while
> > > remote_commit_ts and local_conflicts.commit_ts remain same. Second RI
> > > is 02/01/2025 in DMY format, meaning 2nd january.
> > >
> > > 3)
> > > Now if I do this:
> > > SET DateStyle = 'ISO, MDY'
> > >
> > > postgres=# select conflict_type, remote_commit_ts, replica_identity,
> > > local_conflicts from pg_conflict.pg_conflict_log_16390;
> > > conflict_type | remote_commit_ts |
> > > replica_identity |
> > > local_conflicts
> > > -----------------------+----------------------------------+-----------------------------------+------------------------------------------------------------------------------------------
> > > delete_origin_differs | 2026-10-09 14:15:29.055423+05:30 |
> > > {"k":"2025-12-25 10:00:00+05:30"} |
> > > {"{\"xid\":\"672\",\"commit_ts\":\"2026-10-09T14:15:22.033391+05:30\",\"origin\":null}"}
> > > delete_origin_differs | 2026-10-09 14:17:12.929024+05:30 |
> > > {"k":"02/01/2025 08:00:00 IST"} |
> > > {"{\"xid\":\"678\",\"commit_ts\":\"2026-10-09T14:17:09.326423+05:30\",\"origin\":null}"}
> > > (2 rows)
> > >
> > > See remote_commit_ts changed from previous SQL to ISO, while
> > > replica_identity and local_conflicts.commit_ts are the same.
> > >
> > > If I try to read it as documented (casting it back to original
> > > column's type), I get misleading results:
> > >
> > > postgres=# SET DateStyle = 'ISO, MDY';
> > >
> > > postgres=#
> > > SELECT (local_conflicts[1]->>'commit_ts')::timestamptz as local_ts,
> > > (replica_identity->>'k')::timestamptz as ri_ts
> > > FROM pg_conflict.pg_conflict_log_16390;
> > >
> > > local_ts | ri_ts
> > > ----------------------------------+---------------------------
> > > 2026-10-09 14:15:22.033391+05:30 | 2025-12-25 10:00:00+05:30
> > > 2026-10-09 14:17:09.326423+05:30 | 2025-02-01 08:00:00+05:30
> > > (2 rows)
> > >
> > > See second row for ri_ts, it will now represent '1st Feb' rather than
> > > '2nd January'.
> > >
> > > Shouldn't all three timestamps be in the same format?
> > >
> > > IMO, remote_commit_ts being session-dependent is expected.
> > > The concern is the other two JSON text columns. replica_identity
> > > stores the worker’s DateStyle, so the ->>'k'::timestamptz can
> > > silently return a different value (here 2nd Jan read back as 1st Feb);
> > > while local_conflicts.commit_ts is always ISO. Shouldn’t the recorded
> > > JSON timestamps use a single, stable format (ISO) so they’re
> > > unambiguous and safe to cast back?
> > >
> >
> > What are the values if you don't change the Datestyle manually and use
> > the default or what was used by worker while reading it?
> >
>
>
> With default, values are:
>
> postgres=# SELECT conflict_type,
> remote_commit_ts,
> replica_identity->>'k' AS ri_key_timestamp,
> local_conflicts[1]->>'commit_ts' AS lc_commit_ts
> FROM pg_conflict.pg_conflict_log_16390;
>
> conflict_type | remote_commit_ts |
> ri_key_timestamp | lc_commit_ts
> -----------------------+----------------------------------+---------------------------+----------------------------------
> delete_origin_differs | 2026-10-12 10:47:07.349306+05:30 | 2025-12-25
> 10:00:00+05:30 | 2026-10-12T10:47:04.618018+05:30
> delete_origin_differs | 2026-10-12 10:48:22.419669+05:30 | 2025-01-02
> 08:00:00+05:30 | 2026-10-12T10:48:20.277719+05:30
> (2 rows)
>
> Default DateStyle of Db:
>
> postgres=# show DateStyle;
> DateStyle
> -----------
> ISO, DMY
> (1 row)
>
> Same is used by worker while insert and in same we have read the above data.
>
> ~~
>
> So with the default (ISO), or whenever the reader’s DateStyle matches
> the one the worker used at write time, the values read back correctly;
> the misleading result only appears when the reader’s DateStyle differs
> from the worker’s write-time DateStyle (specifically when the worker
> wrote under a non-ISO style).
>
> ~~
>
> Further observations and analysis:
>
> 1) Apply worker uses current datestyle of DB for replica_identity, and
> then materializes it into text (json col).
>
> 2) local_conflicts always uses ISO because it’s built via row_to_json.
> See JsonEncodeDateTime() → EncodeDateTime(), which is called with
> USE_XSD_DATES; in EncodeDateTime() that falls through the same branch
> as USE_ISO_DATES.
>
> 3) remote_commit_ts is stored as timestamp column rather than text in json.
>
> The one stored as timestamptz (3) will have no issues. Because it’s a
> genuine timestamptz, we always get back the correct instant; only its
> display changes with the reader’s DateStyle.
>
> Both (1) and (2) are frozen as text, but IIUC, only replica_identity
> is at risk.
>
> local_conflicts.commit_ts avoids it by alwyas using ISO (written
> always as YYYY-MM-DD, where month and day are in fixed positions. Thus
> no reader’s DMY/MDY setting can swap them.
>
> replica_identity is the only one that stores the worker’s DateStyle
> into stored text (unknown and un-recoverable later). Different rows
> can be frozen under different styles (config change), so a reader has
> no reliable style to pick while reading.
>
> The fix could be to have the apply worker also use ISO style while
> inserting; similar to row_to_json, and how logical replication’s
> libpqwalreceiver standardizes the wire timestamp values using ISO. See
> libpqrcv_connect using datestyle=ISO.
>

On checking libpqwalreceiver further, I see that subscriber asks
publisher to give data with these 3 GUC settings to remove any
ambiguity:
-c datestyle=ISO -c intervalstyle=postgres -c extra_float_digits=3

We have already analyzed above the impact of 'datestyle' on
'replica_idenity' value. I was wondering if along with 'datestyle',
the other 2 can also impact the correctness of the CLT data? I think
we need to analyze the other two GUCs as well.

thanks
Shveta