Re: Skipping logical replication transactions on subscriber side - Mailing list pgsql-hackers

On Mon, Apr 04, 2022 at 06:55:45PM +0900, Masahiko Sawada wrote:
> On Mon, Apr 4, 2022 at 3:26 PM Noah Misch <noah@leadboat.com> wrote:
> > On Mon, Apr 04, 2022 at 08:20:08AM +0530, Amit Kapila wrote:
> > > How about a comment like: "It has to be kept at 8-byte alignment
> > > boundary so as to be accessed directly via C struct as it uses
> > > TYPALIGN_DOUBLE for storage which has 4-byte alignment on platforms
> > > like AIX."? Can you please suggest a better comment if you don't like
> > > this one?
> >
> > I'd write it like this, though I'm not sure it's an improvement on your words:
> >
> >   When ALIGNOF_DOUBLE==4 (e.g. AIX), the C ABI may impose 8-byte alignment on
> >   some of the C types that correspond to TYPALIGN_DOUBLE SQL types.  To ensure
> >   catalog C struct layout matches catalog tuple layout, arrange for the tuple
> >   offset of each fixed-width, attalign='d' catalog column to be divisible by 8
> >   unconditionally.  Keep such columns before the first NameData column of the
> >   catalog, since packagers can override NAMEDATALEN to an odd number.
> 
> Thanks!
> 
> >
> > The best place for such a comment would be in one of
> > src/test/regress/sql/*sanity*.sql, next to a test written to detect new
> > violations.
> 
> Agreed.
> 
> IIUC in the new test, we would need a new SQL function to calculate
> the offset of catalog columns including padding, is that right? Or do
> you have an idea to do that by using existing functionality?

Something like this:

select
  attrelid::regclass,
  attname,
  array(select typname
        from pg_type t join pg_attribute pa on t.oid = pa.atttypid
        where pa.attrelid = a.attrelid and pa.attnum > 0 and pa.attnum < a.attnum order by pa.attnum) AS types_before,
  (select sum(attlen)
   from pg_type t join pg_attribute pa on t.oid = pa.atttypid
   where pa.attrelid = a.attrelid and pa.attnum > 0 and pa.attnum < a.attnum) AS len_before
from pg_attribute a
join pg_class c on c.oid = attrelid
where attalign = 'd' and relkind = 'r' and attnotnull and attlen <> -1
order by attrelid::regclass::text, attnum;
    attrelid     │   attname    │                types_before                 │ len_before
─────────────────┼──────────────┼─────────────────────────────────────────────┼────────────
 pg_sequence     │ seqstart     │ {oid,oid}                                   │          8
 pg_sequence     │ seqincrement │ {oid,oid,int8}                              │         16
 pg_sequence     │ seqmax       │ {oid,oid,int8,int8}                         │         24
 pg_sequence     │ seqmin       │ {oid,oid,int8,int8,int8}                    │         32
 pg_sequence     │ seqcache     │ {oid,oid,int8,int8,int8,int8}               │         40
 pg_subscription │ subskiplsn   │ {oid,oid,name,oid,bool,bool,bool,char,bool} │         81
(6 rows)

That doesn't count padding, but hazardous column changes will cause a diff in
the output.