Horton

Re: Statistics Import and Export: difference in statistics dumped

Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>

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From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Corey Huinker <corey.huinker@gmail.com>, Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>, Michael Paquier <michael@paquier.xyz>, Nathan Bossart <nathandbossart@gmail.com>, Bruce Momjian <bruce@momjian.us>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Magnus Hagander <magnus@hagander.net>, Stephen Frost <sfrost@snowman.net>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Alvaro Herrera <alvherre@alvh.no-ip.org>, jian he <jian.universality@gmail.com>
Date: 2025-03-05T09:52:56Z
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 Tue, Mar 4, 2025 at 11:45 PM Jeff Davis <pgsql@j-davis.com> wrote:
>
> On Tue, 2025-03-04 at 10:28 +0530, Ashutosh Bapat wrote:
>
> > >
> > > What solution are you suggesting? The only one that comes to mind
> > > is
> > > moving everything to SECTION_POST_DATA, which is possible, but it
> > > seems
> > > like a big design change to satisfy a small detail.
> >
> > We don't have to do that. We can manage it by making statistics of
> > index dependent upon the indexes on the table.
>
> The index relstats are already dependent on the index definition. If
> you have a simple database like:
>
>    CREATE TABLE t(i INT);
>    INSERT INTO t SELECT generate_series(1,10);
>    CREATE INDEX t_idx ON t (i);
>    ANALYZE;
>
> and then you dump it, you get:
>
>
>    ------- SECTION_PRE_DATA -------
>
>    CREATE TABLE public.t ...
>
>    ------- SECTION_DATA -----------
>
>    COPY public.t (i) FROM stdin;
>    ...
>    SELECT * FROM pg_catalog.pg_restore_relation_stats(
>         'version', '180000'::integer,
>         'relation', 'public.t'::regclass,
>         'relpages', '1'::integer,
>         'reltuples', '10'::real,
>         'relallvisible', '0'::integer
>    );
>    ...
>
>    ------- SECTION_POST_DATA ------
>
>    CREATE INDEX t_idx ON public.t USING btree (i);
>    SELECT * FROM pg_catalog.pg_restore_relation_stats(
>         'version', '180000'::integer,
>         'relation', 'public.t_idx'::regclass,
>         'relpages', '2'::integer,
>         'reltuples', '10'::real,
>         'relallvisible', '0'::integer
>    );
>
> (section annotations added for clarity)
>
> There is no problem with the index relstats, because they are already
> dependent on the index definition, and will be restored after the
> CREATE INDEX.
>
> The issue is when the table's restored relstats are different from what
> CREATE INDEX calculates, and then the CREATE INDEX overwrites the
> table's just-restored relation stats. The easiest way to see this is
> when restoring with --no-data, because CREATE INDEX will see an empty
> table and overwrite the table's restored relstats with zeros.
>
> If we view this issue as a dependency problem, then we'd have to make
> the *table relstats* depend on the *index definition*. If a table has
> any indexes, the relstats would need to go after the last index
> definition, effectively moving most relstats to SECTION_POST_DATA. The
> table's attribute stats would not be dependent on the index definition
> (because CREATE INDEX doesn't touch those), so they could stay in
> SECTION_DATA. And if the table doesn't have any indexes, then its
> relstats could also stay in SECTION_DATA. But then we have a mess, so
> we might as well just put all stats in SECTION_POST_DATA.
>
> But I don't see it as a dependency problem. When I look at the above
> SQL, it reads nicely to me and there's no obvious problem with it.

Thanks for explaining it. I

>
> If we want stats to be stable, we need some kind of mode to tell the
> server not to apply these kind of helpful optimizations, otherwise the
> issue will resurface in some form no matter what we do with pg_dump. We
> could invent a new mode, but autovacuum=off seems close enough to me.

Hmm. Updating the statistics without consuming more CPU is more
valuable when autovacuum is off it improves query plans with no extra
efforts. But if adding a new mode is some significant work, riding it
on top of autovacuum=off might ok. It's not documented either way, so
we could change that behaviour later if we find it troublesome.

-- 
Best Wishes,
Ashutosh Bapat