On Mon, Apr 04, 2022 at 06:55:45PM +0900, Masahiko Sawada wrote:
> On Mon, Apr 4, 2022 at 3:26 PM Noah Misch <[email protected]> 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.
