Re: Skipping logical replication transactions on subscriber side

On Wed, Jun 22, 2022 at 09:50:02AM -0400, Robert Haas wrote:
> On Wed, Jun 22, 2022 at 12:28 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > Here's how I rank the options, from most-preferred to least-preferred:
> >
> > 1. Put new eight-byte fields at the front of each catalog, when in doubt.
> > 2. On systems where double alignment differs from int64 alignment, require
> > NAMEDATALEN%8==0. Upgrading to v16 would require dump/reload for AIX users
> > changing NAMEDATALEN to conform to the new restriction.
> > 3. Introduce new typalign values. Upgrading to v16 would require dump/reload
> > for all AIX users.
> > 4. De-support AIX.
> > 5. From above, "Somehow forcing values that are sometimes 4-byte aligned and
> > sometimes 8-byte aligned to be 8-byte alignment on all platforms".
> > Upgrading to v16 would require dump/reload for all AIX users.
> > 6. Require -qalign=natural on AIX. Upgrading to v16 would require dump/reload
> > and possible system library rebuilds for all AIX users.
> >
> > I gather (1) isn't at the top of your ranking, or you wouldn't have written
> > in. What do you think of (2)?
>
> (2) pleases me in the sense that it seems to inconvenience very few
> people, perhaps no one, in order to avoid inconveniencing a larger
> number of people. However, it doesn't seem sufficient.

Here's a more-verbose description of (2), with additions about what it does
and doesn't achieve:

2. On systems where double alignment differs from int64 alignment, require
NAMEDATALEN%8==0. Modify the test from commits 79b716c and c1da0ac to stop
treating "name" fields specially. The test will still fail for AIX
compatibility violations, but "name" columns no longer limit your field
position candidates like they do today (today == option (1)). Upgrading to
v16 would require dump/reload for AIX users changing NAMEDATALEN to conform
to the new restriction. (I'm not sure pg_upgrade checks NAMEDATALEN
compatibility, but it should require at least one of: same NAMEDATALEN, or
absence of "name" columns in user tables.)

> If I understand
> correctly, even a catalog that includes no NameData column can have a
> problem.

Correct.

On Wed, Jun 22, 2022 at 10:39:20AM -0400, Tom Lane wrote:
> It appears that what we've got on AIX is that typalign 'd' overstates the
> actual alignment requirement for 'double', which is safe from the SIGBUS
> angle.

On AIX, typalign='d' states the exact alignment requirement for 'double'. It
understates the alignment requirement for int64_t.

> I don't think we want rules exactly, what we need is mechanical verification
> that the field orderings in use are safe.

Commits 79b716c and c1da0ac did that.