Horton

Re: Extended Statistics set/restore/clear functions.

Michael Paquier <michael@paquier.xyz>

View thread
From: Michael Paquier <michael@paquier.xyz>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: jian he <jian.universality@gmail.com>, Tomas Vondra <tomas@vondra.me>, pgsql-hackers@lists.postgresql.org, tgl@sss.pgh.pa.us
Date: 2025-10-22T23:46:27Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add test doing some cloning of extended statistics data

  2. Add test for pg_restore_extended_stats() with multiranges

  3. Add support for "mcv" in pg_restore_extended_stats()

  4. Include extended statistics data in pg_dump

  5. Add support for "dependencies" in pg_restore_extended_stats()

  6. Add test for MAINTAIN permission with pg_restore_extended_stats()

  7. Add pg_restore_extended_stats()

  8. Add routine to free MCVList

  9. Improve pg_clear_extended_stats() with incorrect relation/stats combination

  10. Add pg_clear_extended_stats()

  11. Introduce routines to validate and free MVNDistinct and MVDependencies

  12. Fix typo in stat_utils.c

  13. Move attribute statistics functions to stat_utils.c

  14. Improve error messages of input functions for pg_dependencies and pg_ndistinct

  15. Improve test output of extended statistics for ndistinct and dependencies

  16. Fix some compiler warnings

  17. Add input function for data type pg_dependencies

  18. Add input function for data type pg_ndistinct

  19. Rework output format of pg_dependencies

  20. Rework output format of pg_ndistinct

  21. Fix comments of output routines for pg_ndistinct and pg_dependencies

  22. Move code specific to pg_dependencies to new file

  23. Move code specific to pg_ndistinct to new file

  24. Document some structures in attribute_stats.c

  25. Fix FATAL message for invalid recovery timeline at beginning of recovery

On Wed, Oct 22, 2025 at 02:55:31PM +0300, Corey Huinker wrote:
>> Do you have some numbers regarding the increase in size this generates
>> for the catalogs?
> 
> Sorry, I don't understand. There shouldn't be any increase inside the
> catalogs as the internal storage of the datatypes hasn't changed, so I can
> only conclude that you're referring to something else.

The new format meant more characters, perhaps I've just missed
something while quickly testing the patch..  Anyway, that's OK at this
stage.

> The equivalent structures in attribute_stats.c will need documenting too.

Right.  This sounds like a separate patch to me, impacting HEAD.

> Right now we have a situation where the vast majority of databases can
> carry forward all of their stats via pg_upgrade, except for those databases
> that have extended stats. The trouble is, most customers don't know if
> their database uses extended statistics or not, and those that do are in
> for some bad query plans if they haven't run vacuumdb --missing-stats-only.
> Explaining that to customers is complicated, especially when most of them
> do not know what extended stats are, let alone whether they have them. It
> would be a lot simpler to just say "all stats are carried over on upgrade",
> and vacuumdb becomes unnecessary, making upgrades one step simpler as well.

Okay.

> Given that, I think that the admittedly ugly transformation is worth it,
> and sequestering it inside pg_dump is the smallest footprint it can have.
> Earlier in this thread I posted some functions that did the translation
> from the existing formats to the proposed new formats. We could include
> those as new system functions, and that would make the dump code very
> simple. Having said that, I don't know that there would be use for those
> functions except inside pg_dump, hence the decision to do the transforms
> right in the dump query.

I'd prefer the new format.  One killer pushing in favor of the new
format that you are making upthread in favor of is that it makes much
easier the viewing, editing and injecting of these stats.  It's the
part of the patch where we would need Tomas' input on the matter
before deciding anything, I guess, as primary author of the original
facilities.  My view of the problem is just one opinion.

> If the format translation is a barrier to fetching existing extended stats,
> then I'd be more inclined to keep the existing pg_ndistinct and
> pg_dependencies data formats as they are now.

Not necessarily, it can be possible to also take that in multiple
steps rather than a single one:
- First do the basics in v19 with the new format.
- Raise the bar to older versions.
--
Michael