Re: Proposal: Conflict log history table for Logical Replication

Hi Nisha,

Thank you very much for the guidance and the pointers to the older thread,

Please find some replies inline,

On Wed, Sep 30, 2026 at 11:05 AM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com>
wrote:

> On Tue, Sep 29, 2026 at 5:56 PM Narayanan Venkateswaran
> <narayananvpostgres(at)gmail(dot)com> wrote:
> >
> > Thank you very much for the excellent work. I looked at the patch v77,
> >
>
> Hi Narayanan, thanks for reviewing it.
>
> > The code decides replica_identity_full using the following logic (in
> conflict.c),
> >
> > if (!TupIsNull(searchslot))
> > {
> > Oid replica_index = GetRelationIdentityOrPK(rel);
> >
> > /*
> > * If the table has a valid replica identity index, build the index
> > * JSON datum from key value. Otherwise, in REPLICA IDENTITY FULL
> > * cases, set replica_identity_full to true and leave replica_identity
> > * NULL to avoid serializing full tuples that could exceed memory
> > * allocation limits.
> > */
> > if (OidIsValid(replica_index))
> > {
> > values[attno++] = BoolGetDatum(false);
> > values[attno++] = build_index_key_json(rel,
> > replica_index,
> > searchslot,
> > &omitted);
> > }
> > else
> > {
> > values[attno++] = BoolGetDatum(true);
> > nulls[attno++] = true;
> > }
> > }
> > else
> > {
> > nulls[attno++] = true;
> > nulls[attno++] = true;
> > }
> >
> > In PostgreSQL catalogs (pg_class.relreplident), a table's replica can be
> one of four values:
> >
> > 'd' = REPLICA_IDENTITY_DEFAULT: Use PK index if one exists. If the table
> has no PK, it has no index and is NOT FULL.
> > 'n' = REPLICA_IDENTITY_NOTHING: No replica identity.
> > 'i' = REPLICA_IDENTITY_INDEX: Explicit unique index.
> > 'f' = REPLICA_IDENTITY_FULL: The entire tuple is the identity.
> >
> > If a subscriber relation has REPLICA IDENTITY DEFAULT without a primary
> key (or REPLICA IDENTITY NOTHING) GetRelationIdentityOrPK() returns
> InvalidOid. In this case, the code sets replica_identity_full = true.
> >
>
> I think there may be some misunderstanding about what the
> replica_identity_full column actually stores. This question was also
> raised earlier; please see [1] and the discussion that followed. This
> field indicates the subscriber’s actual search method.
>

Thank you for clarifying the intended design and also the pointer to the
old thread.

I understand now that the intention for `replica_identity_full` is to
indicate whether the conflicting row was located via a specific replica key
index (`false`) versus a full-tuple search (`true`), rather than reflecting
the DDL catalog property (pg_class.relreplident).

>
> > However, the SGML docs update in the patch states that
> replica_identity_full "is NULL when replica identity information is not
> applicable".
> >
> > The only way replica_identity_full can ever be set to NULL is if the
> execution enters the outer else block: when TupIsNull(searchslot) is true
> (i.e., searchslot is NULL or empty). However, it looks like this slot
> contains the incoming row data sent by the publisher. It is always
> populated and never null.
> >
> > Because searchslot is never null, the outer else block is never
> executed. The code will never set replica_identity_full to NULL.
> >
> > I think it is better to explicitly check for REPLICA_IDENTITY_FULL in an
> else if block, something like the below,
> >
> > else if (rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL)
> > {
> > values[attno++] = BoolGetDatum(true);
> > nulls[attno++] = true;
> > }
> >
>
> As per [1], replica_identity_full value is determined independently of
> relreplident, so I don't think we need this else-if branch.
>

However, I have the following question related to the following doc entry,

+ <row>
+ <entry><literal>replica_identity_full</literal></entry>
+ <entry><type>boolean</type></entry>
+ <entry>Indicates whether the conflicting relation uses
<literal>REPLICA IDENTITY FULL</literal> (<literal>true</literal>) or a
replica identity index (<literal>false</literal>). This is
<literal>NULL</literal> when replica identity information is not
applicable.</entry>
+ </row>

The doc states it is "NULL when replica identity information is not
applicable". However, in insert_conflict_log_tuple(), replica_identity_full
is only set to NULL if TupIsNull(searchslot) is true. Since searchslot
(remoteslot) is always populated for all currently logged conflicts, the
outer else block is never reached and replica_identity_full is never NULL.
Should the documentation be updated to remove the reference to NULL, or is
there a case where searchslot can be empty ?

>
> [I would request you to please reply inline to keep the discussion
> relevant and easier to follow.]
>

Really sorry, my humble apologies for the inconvenience.

>
> [1]
> https://www.postgresql.org/message-id/CAFiTN-u8xH%2BLVVNx8OJxFnub5eHTWw9v7sCcffXtPKKQ1CG2Gw%40mail.gmail.com
> --
> Thanks,
> Nisha
>

Thank you,
Narayanan