Horton

Re: Statistics Import and Export

Corey Huinker <corey.huinker@gmail.com>

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From: Corey Huinker <corey.huinker@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Magnus Hagander <magnus@hagander.net>, Jeff Davis <pgsql@j-davis.com>, Stephen Frost <sfrost@snowman.net>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>
Date: 2024-03-31T22:37:28Z
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. Change pg_dump default for statistics export.

  2. pg_dump: Adjust reltuples from 0 to -1 for dumps of older versions.

  3. vacuumdb: Don't skip empty relations in --missing-stats-only mode.

  4. pg_dump: Fix query for gathering attribute stats on older versions.

  5. Prevent redeclaration of typedef TocEntry.

  6. Remove unused function parameters in pg_backup_archiver.c.

  7. pg_dump: Retrieve attribute statistics in batches.

  8. pg_dump: Reduce memory usage of dumps with statistics.

  9. Skip second WriteToc() call for custom-format dumps without data.

  10. Add relallfrozen to pg_dump statistics.

  11. Matview statistics depend on matview data.

  12. Add pg_dump --with-{schema|data|statistics} options.

  13. Stats: use schemaname/relname instead of regclass.

  14. CREATE INDEX: do update index stats if autovacuum=off.

  15. Don't convert to and from floats in pg_dump.

  16. CREATE INDEX: don't update table stats if autovacuum=off.

  17. Organize and deduplicate statistics import tests.

  18. Address stats export review comments.

  19. Address stats import review comments.

  20. Add relallfrozen to pg_class

  21. Fix pg_strtof() to not crash on NULL endptr.

  22. Use attnum to identify index columns in pg_restore_attribute_stats().

  23. pg_dump: prepare attribute stats query.

  24. Avoid unnecessary relation stats query in pg_dump.

  25. Remove redundant pg_set_*_stats() variants.

  26. Do not use in-place updates for statistics import.

  27. Fix confusion about data type of pg_class.relpages and relallvisible.

  28. Documentation fixups for dumping statistics.

  29. Trial fix for old cross-version upgrades.

  30. Transfer statistics during pg_upgrade.

  31. Lock table in ShareUpdateExclusive when importing index stats.

  32. Use in-place updates for pg_restore_relation_stats().

  33. Improve error message for replication of generated columns.

  34. pg_dump: Add dumpSchema and dumpData derivative flags.

  35. Disallow modifying statistics on system columns.

  36. Add missing CommandCounterIncrement() in stats import functions.

  37. Add functions pg_restore_relation_stats(), pg_restore_attribute_stats().

  38. Documentation fixup.

  39. Add functions pg_set_attribute_stats() and pg_clear_attribute_stats().

  40. Change pg_*_relation_stats() functions to return type to void.

  41. Disable autovacuum for tables in stats import tests.

  42. Allow pg_set_relation_stats() to set relpages to -1.

  43. Fixup for pg_set_relation_stats().

  44. Create functions pg_set_relation_stats, pg_clear_relation_stats.

  45. Add memory/disk usage for Window aggregate nodes in EXPLAIN.

  46. Improve performance of dumpSequenceData().

  47. Add INJECTION_POINT_CACHED() to run injection points directly from cache

  48. Improve performance of binary_upgrade_set_pg_class_oids().

  49. Improve assertion in mdwritev()

  50. CREATE INDEX: do not update stats during binary upgrade.

  51. Redefine pg_class.reltuples to be -1 before the first VACUUM or ANALYZE.

On Sun, Mar 31, 2024 at 2:41 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Corey Huinker <corey.huinker@gmail.com> writes:
> > Having given this some thought, I'd be inclined to create a view,
> > pg_stats_missing, with the same security barrier as pg_stats, but looking
> > for tables that lack stats on at least one column, or lack stats on an
> > extended statistics object.
>
> The week before feature freeze is no time to be designing something
> like that, unless you've abandoned all hope of getting this into v17.
>

It was a response to the suggestion that there be some way for
tools/automation to read the status of stats. I would view it as a separate
patch, as such a view would be useful now for knowing which tables to
ANALYZE, regardless of whether this patch goes in or not.


> There's a bigger issue though: AFAICS this patch set does nothing
> about dumping extended statistics.  I surely don't want to hold up
> the patch insisting that that has to happen before we can commit the
> functionality proposed here.  But we cannot rip out pg_upgrade's
> support for post-upgrade ANALYZE processing before we do something
> about extended statistics, and that means it's premature to be
> designing any changes to how that works.  So I'd set that whole
> topic on the back burner.
>

So Extended Stats _were_ supported by earlier versions where the medium of
communication was JSON. However, there were several problems with adapting
that to the current model where we match params to stat types:

* Several of the column types do not have functional input functions, so we
must construct the data structure internally and pass them to
statext_store().
* The output functions for some of those column types have lists of
attnums, with negative values representing positional expressions in the
stat definition. This information is not translatable to another system
without also passing along the attnum/attname mapping of the source system.

At least three people told me "nobody uses extended stats" and to just drop
that from the initial version. Unhappy with this assessment, I inquired as
to whether my employer (AWS) had some internal databases that used extended
stats so that I could get good test data, and came up with nothing, nor did
anyone know of customers who used the feature. So when the fourth person
told me that nobody uses extended stats, and not to let a rarely-used
feature get in the way of a feature that would benefit nearly 100% of
users, I dropped it.


> It's possible that we could drop the analyze-in-stages recommendation,
> figuring that this functionality will get people to the
> able-to-limp-along level immediately and that all that is needed is a
> single mop-up ANALYZE pass.  But I think we should leave that till we
> have a bit more than zero field experience with this feature.


It may be that we leave the recommendation exactly as it is.

Perhaps we enhance the error messages in pg_set_*_stats() to indicate what
command would remediate the issue.