Re: Setting pd_lower in GIN metapage

On Thu, Sep 7, 2017 at 5:49 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> writes:
>> On 2017/08/22 9:39, Michael Paquier wrote:
>>> On Tue, Jun 27, 2017 at 4:56 PM, Amit Langote
>>> <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>>> I updated brin_mask() and spg_mask() in the attached updated patches so
>>>> that they consider meta pages as containing unused space.
>
> I looked briefly at these patches. I'm not sure that it's safe for the
> mask functions to assume that meta pages always have valid pd_lower.
> What happens when replaying log data concerning an old index that doesn't
> have that field filled?

There will be inconsistency between the pages, and the masking check
will complain. My point here is that wal_consistency_checking is
primarily used by developers on newly-deployed clusters to check WAL
consistency by using installcheck. So an upgraded cluster would see
diffs because of that, but I would think that nobody would really face
them. Perhaps we should document this point for wal_consistency_check?
Getting the patch discussed on this thread into v10 would have saved
the day here, but this boat has sailed already.
--
Michael