Re: [PATCH] Use indexes on the subscriber when REPLICA IDENTITY is full on the publisher

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2023-01-07 13:50:04 -0500, Tom Lane wrote:
>> Also ... maybe I am missing something, but is REPLICA IDENTITY FULL
>> sanely defined in the first place? It looks to me that
>> RelationFindReplTupleSeq assumes without proof that there is a unique
>> full-tuple match, but that is only reasonable to assume if there is at
>> least one unique index (and maybe not even then, if nulls are involved).

> If the table definition match between publisher and standby, it doesn't matter
> which tuple is updated, if all columns are used to match. Since there's
> nothing distinguishing two rows with all columns being equal, it doesn't
> matter which we update.

Yeah, but the point here is precisely that they might *not* match;
for example there could be extra columns in the subscriber's table.
This may be largely a documentation problem, though --- I think my
beef is mainly that there's nothing in our docs explaining the
semantic pitfalls of FULL, we only say "it's slow".

Anyway, to get back to the point at hand: if we do have a REPLICA IDENTITY
FULL situation then we can make use of any unique index over a subset of
the transmitted columns, and if there's more than one candidate index
it's unlikely to matter which one we pick. Given your comment I guess
we have to also compare the non-indexed columns, so we can't completely
convert the FULL case to the straight index case. But still it doesn't
seem to me to be appropriate to use the planner to find a suitable index.

regards, tom lane