Re: Proposal: Conflict log history table for Logical Replication

On Sat, 6 Dec 2025 at 20:36, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Fri, Dec 5, 2025 at 10:39 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Thu, Dec 4, 2025 at 8:05 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> > >
> > > On Wed, 3 Dec 2025 at 16:57, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > > On Wed, Dec 3, 2025 at 9:49 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > relid | 16391
> > > > > > schemaname | public
> > > > > > relname | conf_tab
> > > > > > conflict_type | multiple_unique_conflicts
> > > > > > remote_xid | 761
> > > > > > remote_commit_lsn | 0/01761400
> > > > > > remote_commit_ts | 2025-12-02 15:02:07.045935+00
> > > > > > remote_origin | pg_16406
> > > > > > key_tuple |
> > > > > > remote_tuple | {"a":2,"b":3,"c":4}
> > > > > > local_conflicts |
> > > > > > {"{\"xid\":\"773\",\"commit_ts\":\"2025-12-02T15:02:00.640253+00:00\",\"origin\":\"\",\"tuple\":{\"a\":2,\"b\":2,\"c\":2}}","{\"xid\":\"
> > > > > > 773\",\"commit_ts\":\"2025-12-02T15:02:00.640253+00:00\",\"origin\":\"\",\"tuple\":{\"a\":3,\"b\":3,\"c\":3}}","{\"xid\":\"773\",\"commit_ts\":\"2025-12-02T
> > > > > > 15:02:00.640253+00:00\",\"origin\":\"\",\"tuple\":{\"a\":4,\"b\":4,\"c\":4}}"}
> > > > > >
> > > > >
> > > > > Thanks, it looks good. For the benefit of others, could you include a
> > > > > brief note, perhaps in the commit message for now, describing how to
> > > > > access or read this array column? We can remove it later.
> > > >
> > > > Thanks, okay, temporarily I have added in a commit message how we can
> > > > fetch the data from the JSON array field. In next version I will add
> > > > a test to get the conflict stored in conflict log history table and
> > > > fetch from it.
> > >
> > > I noticed that the table structure can get changed by the time the
> > > conflict record is prepared. In ReportApplyConflict(), the code
> > > currently prepares the conflict log tuple before deciding whether the
> > > insertion will be immediate or deferred:
> > > + /* Insert conflict details to conflict log table. */
> > > + if (conflictlogrel)
> > > + {
> > > + /*
> > > + * Prepare the conflict log tuple. If the error level
> > > is below ERROR,
> > > + * insert it immediately. Otherwise, defer the
> > > insertion to a new
> > > + * transaction after the current one aborts, ensuring
> > > the insertion of
> > > + * the log tuple is not rolled back.
> > > + */
> > > + prepare_conflict_log_tuple(estate,
> > > +
> > > relinfo->ri_RelationDesc,
> > > +
> > > conflictlogrel,
> > > + type,
> > > + searchslot,
> > > +
> > > conflicttuples,
> > > + remoteslot);
> > > + if (elevel < ERROR)
> > > + InsertConflictLogTuple(conflictlogrel);
> > > +
> > > + table_close(conflictlogrel, RowExclusiveLock);
> > > + }
> > >
> > > If the conflict history table defintion is changed just before
> > > prepare_conflict_log_tuple, the tuple creation will crash:
> > > Program received signal SIGSEGV, Segmentation fault.
> > > 0x00005a342e01df4f in VARATT_CAN_MAKE_SHORT (PTR=0x4000) at
> > > ../../../../src/include/varatt.h:419
> > > 419 return VARATT_IS_4B_U(PTR) &&
> > > (gdb) bt
> > > #0 0x00005a342e01df4f in VARATT_CAN_MAKE_SHORT (PTR=0x4000) at
> > > ../../../../src/include/varatt.h:419
> > > #1 0x00005a342e01e5ed in heap_compute_data_size
> > > (tupleDesc=0x7ab405e5dda8, values=0x7ffd7af3ad20,
> > > isnull=0x7ffd7af3ad15) at heaptuple.c:239
> > > #2 0x00005a342e0200dd in heap_form_tuple
> > > (tupleDescriptor=0x7ab405e5dda8, values=0x7ffd7af3ad20,
> > > isnull=0x7ffd7af3ad15) at heaptuple.c:1158
> > > #3 0x00005a342e55e8c2 in prepare_conflict_log_tuple
> > > (estate=0x5a3467944530, rel=0x7ab405e594e8,
> > > conflictlogrel=0x7ab405e5da88, conflict_type=CT_INSERT_EXISTS,
> > > searchslot=0x0,
> > > conflicttuples=0x5a3467942da0, remoteslot=0x5a346792e498) at conflict.c:936
> > > #4 0x00005a342e55cea6 in ReportApplyConflict (estate=0x5a3467944530,
> > > relinfo=0x5a346792e778, elevel=21, type=CT_INSERT_EXISTS,
> > > searchslot=0x0, remoteslot=0x5a346792e498,
> > > conflicttuples=0x5a3467942da0) at conflict.c:168
> > > #5 0x00005a342e348c35 in CheckAndReportConflict
> > > (resultRelInfo=0x5a346792e778, estate=0x5a3467944530,
> > > type=CT_INSERT_EXISTS, recheckIndexes=0x5a3467942648, searchslot=0x0,
> > > remoteslot=0x5a346792e498) at execReplication.c:793
> > >
> > > This can be reproduced by the following steps:
> > > CREATE PUBLICATION pub;
> > > CREATE SUBSCRIPTION sub ... WITH (conflict_log_table = 'conflict');
> > > ALTER TABLE conflict RENAME TO conflict1:
> > > CREATE TABLE conflict(c1 varchar, c2 varchar);
> > > -- Cause a conflict, this will crash while trying to prepare the
> > > conflicting tuple
> >
> > Yeah while it is allowed to drop or alter the conflict log table, it
> > should not seg fault, IMHO error is acceptable as per the initial
> > discussion, so I will look into this and tighten up the logic so that
> > it will throw an error whenever it can not insert into the conflict
> > log table.
>
> I was thinking about the solution that we need to do if table
> definition is changed, one option is whenever we try to prepare the
> tuple after acquiring the lock we can validate the table definition if
> this doesn't qualify the standard conflict log table schema we can
> ERROR out. IMHO that should not be an issue as we are only doing this
> in conflict logging.

Should we emit a warning instead of error, to stay consistent with the
other exception case where a warning is raised when the conflict log
table does not exist?
+ /* Conflict log table is dropped or not accessible. */
+ if (conflictlogrel == NULL)
+ ereport(WARNING,
+ (errcode(ERRCODE_UNDEFINED_TABLE),
+ errmsg("conflict log table \"%s.%s\"
does not exist",
+
get_namespace_name(nspid), conflictlogtable)));

Regards,
Vignesh