Re: pg_amcheck contrib application

On Tue, Mar 16, 2021 at 12:51 PM Mark Dilger
<mark(dot)dilger(at)enterprisedb(dot)com> wrote:
> Yeah, that looks related:
>
> regression=# select attname, attlen, attnum, attalign from pg_attribute where attrelid = 2619 and attname like 'stavalue%';
> attname | attlen | attnum | attalign
> ------------+--------+--------+----------
> stavalues1 | -1 | 27 | d
> stavalues2 | -1 | 28 | d
> stavalues3 | -1 | 29 | d
> stavalues4 | -1 | 30 | d
> stavalues5 | -1 | 31 | d
> (5 rows)
>
> It shows them all has having attalign = 'd', but for some array types the alignment will be 'i', not 'd'. So it's lying a bit about the contents. But I'm now confused why this caused problems on the two hosts where integer and double have the same alignment? It seems like that would be the one place where the bug would not happen, not the one place where it does.

Wait, so the value in the attalign column isn't the alignment we're
actually using? I can understand how we might generate tuples like
that, if we pass the actual type to construct_array(), but shouldn't
we then get garbage when we deform the tuple? I mean,
heap_deform_tuple() is going to get the alignment from the tuple
descriptor, which is a table property. It doesn't (and can't) know
what type of value is stored inside any of these fields for real,
right?

In other words, isn't this actually corruption, caused by a bug in our
code, and how have we not noticed it before now?

--
Robert Haas
EDB: http://www.enterprisedb.com