Re: Proposal: Conflict log history table for Logical Replication

On Fri, May 1, 2026 at 11:46 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Thu, Apr 30, 2026 at 10:40 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Wed, Apr 29, 2026 at 12:34 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Apr 29, 2026 at 11:50 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > > On Tue, Apr 28, 2026 at 7:53 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > > > > 2.
> > > > > > +typedef enum ConflictLogDest
> > > > > > +{
> > > > > > + /* Log conflicts to the server logs */
> > > > > > + CONFLICT_LOG_DEST_LOG = 1 << 0, /* 0x01 */
> > > > > > +
> > > > > > + /* Log conflicts to an internally managed conflict log table */
> > > > > > + CONFLICT_LOG_DEST_TABLE = 1 << 1, /* 0x02 */
> > > > > > +
> > > > > > + /* Convenience bitmask for all supported destinations */
> > > > > > + CONFLICT_LOG_DEST_ALL = (CONFLICT_LOG_DEST_LOG | CONFLICT_LOG_DEST_TABLE)
> > > > > > +} ConflictLogDest;
> > > > > > +
> > > > > > +/*
> > > > > > + * Array mapping for converting internal enum to string.
> > > > > > + */
> > > > > > +static const char *const ConflictLogDestNames[] = {
> > > > > > + [CONFLICT_LOG_DEST_LOG] = "log",
> > > > > > + [CONFLICT_LOG_DEST_TABLE] = "table",
> > > > > > + [CONFLICT_LOG_DEST_ALL] = "all"
> > > > > > +};
> > > > > >
> > > > > > Defining an array this way could be an Array size issue. Actually the
> > > > > > array has just three elements so the last element should be at
> > > > > > ConflictLogDestNames[2] but if we go by the above definition, it will
> > > > > > be ConflictLogDestNames[3]. Can we define by referring the following
> > > > > > existing way:
> > > >
> > > > I was analyzing this because I remember we were initially using the
> > > > format you suggested and switched to the bit format to enable direct
> > > > bitwise operations elsewhere. I think Peter suggested that [1], and
> > > > the argument was that the bitwise operation is easy if we represent
> > > > them as a bit. Also, since we would not have too many options, the
> > > > array size shouldn't be an issue. But I understand your point: adding
> > > > more elements will cause the array size to grow very fast as this is
> > > > using sparse array. Let's see what others think about this, and then
> > > > we can decide whether to change it back?
> > > >
> > >
> > > The benefit of the current approach is that checking whether the
> > > destination is TABLE becomes straightforward:
> > >
> > > IsSet(opts.conflictlogdest,CONFLICT_LOG_DEST_TABLE)
> > >
> > > if we go by regular enum values (simialr to XLogSource), then it will be:
> > >
> > > if (opts.logdest == CONFLICT_LOG_DEST_TABLE ||
> > > opts.logdest == CONFLICT_LOG_DEST_ALL)
> >
> > Right
> >
> > > For ease of extending the enum and its corresponding text mappings, my
> > > personal preference is still the regular (non-bitwise) enum approach.
> >
> > Yeah, that's my personal preference too. But Peter had strong stand
> > on keeping as bitwise so that we can directly use
> > IsSet(opts.conflictLogDest, CONFLICT_LOG_DEST_TABLE) operations.
> > Since this array shouldn't have many options, a sparse array is not an
> > issue. So lets see what @Peter Smith has to say here and then we can
> > build a concensus on this.
> >
> > > But if we anticipate adding more destination options in the future
> > > that would be covered by ALL, checking for those in code could lead to
> > > growing chains of OR conditions, whereas the bitwise approach scales
> > > more cleanly in that respect. So I think the choice depends on what
> > > kinds of future extensions we expect.
> > >
> > > Do we have plans to add more options that would naturally fall under
> > > ALL? Or do we instead expect additions that are mutually exclusive;
> > > for example, splitting CONFLICT_LOG_DEST_LOG into something like
> > > CONFLICT_LOG_DEST_JSON_LOG and CONFLICT_LOG_DEST_TEXT_LOG, which may
> > > not make sense to group under ALL in the same way?
> >
> > Currently, I haven't considered which options would naturally fall
> > under "ALL." Perhaps if we plan targets other than logs and files,
> > those might also fall under "ALL."
>
> I have fixed all the reported comments except these four.
> 1. I'm changing the ConflictLogDest enum from bitmap to integer. I can
> revert this in the next version but I want to see Peter's opinion
> first, as he suggested using a bitmap to easily apply bitwise
> operators.
>

Sorry for the delay in responding. I have been away.

Yes, I recall thinking bitmaps were a tidy way of checking if a CLT
was required, just by:
IsSet(opts.conflictlogdest,CONFLICT_LOG_DEST_TABLE)

IMO, "all" is not really a discrete target value... it meant more like
"a combination of all the other ones". That is why bitmaps felt like a
better fit to me.

Of course, then you will have the (not very) sparse
designated-initializer array of names that some people objected to:
+static const char *const ConflictLogDestNames[] = {
+ [CONFLICT_LOG_DEST_LOG] = "log",
+ [CONFLICT_LOG_DEST_TABLE] = "table",
+ [CONFLICT_LOG_DEST_ALL] = "all"
+};

TBH, I did not think the sparse array posed any real problem because
even if there were 5 target values (which is way more than I could
imagine it growing to) that would still only be a sparse array of 2^5
elements which seemed hardly worth worrying about.

Anyway, it is fine by me if you want to revert to a plain enum. The
code of CreateSubscription/AlterSubscription becomes a bit clunkier
now having to check CONFLICT_LOG_DEST_ALL, but it's OK.

======
Kind Regards,
Peter Smith.
Fujitsu Australia