Re: Extended Statistics set/restore/clear functions.[

On Fri, Oct 31, 2025 at 09:22:55PM +0100, Tomas Vondra wrote:
> Sorry for not paying much attention to this thread ...

It was PGEU last week, it's not surprising knowing that you are in..
Europe :)

> My opinion is that we should both use the new format and keep the
> pg_dump code to allow upgrading from older pre-19 versions.

Okay, thanks for the input.

> There really is nothing special about the current format - I should have
> used JSON (or any other established format) from the beginning. But I
> only saw that as human-readable version of ephemeral data, it didn't
> occur to me we'll use this to export/import stats cross versions. So if
> we need to adjust that to make new use cases more convenient, let's bite
> the bullet now.
>
> If doing both is too complex / ugly, I think the pg_upgrade capability
> is more valuable. I'd rather keep the old, less convenient format to
> have pg_upgrade support for all versions.

Hmm. I have been eyeing at the dump code once again, and it's not
that bad on a second lookup, so I'd be OK to incorporate the whole in
a review.

> Otherwise users may not benefit from this pg_upgrade feature for a
> couple more years. Plenty of users delay upgrading until the EOL gets
> close, and so might be unable to dump/restore extended stats for the
> next ~5 years.

Okay.

I still think that we should split things a bit more in the patch set.
Corey has sent me a proposal in this direction, where most of the
entries can be reviewed and potentially applied separately:
1. pg_ndistinct output function change.
2. pg_ndistinct input function addition.
3. pg_dependencies output function change
4. pg_dependencies input function
5. Expose attribute statistics function and rename them attstat_* or
statatt_*
6. pg_restore_extended_stats
7. pg_dump with no ability to fetch old-format
pg_ndistinct/pg_dependences.
8. pg_dump working back as far as possible

I am not completely sold about the changes in the input/output
functions, but I'd be happy to consider more options with a rebased
patch set (with the dump issue on older clusters issue, while on it).
--
Michael