Horton

Re: Statistics Import and Export

Corey Huinker <corey.huinker@gmail.com>

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From: Corey Huinker <corey.huinker@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Stephen Frost <sfrost@snowman.net>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>
Date: 2024-03-21T19:10:42Z
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.

>
> How about just some defaults then? Many of them have a reasonable
> default, like NULL or an empty array. Some are parallel arrays and
> either both should be specified or neither (e.g.
> most_common_vals+most_common_freqs), but you can check for that.
>

+1
Default NULL has been implemented for all parameters after n_distinct.


>
> > > Why are you calling checkCanModifyRelation() twice?
> >
> > Once for the relation itself, and once for pg_statistic.
>
> Nobody has the privileges to modify pg_statistic except superuser,
> right? I thought the point of a privilege check is that users could
> modify statistics for their own tables, or the tables they maintain.
>

In which case wouldn't the checkCanModify on pg_statistic would be a proxy
for is_superuser/has_special_role_we_havent_created_yet.



>
> >
> > I can see making it void and returning an error for everything that
> > we currently return false for, but if we do that, then a statement
> > with one pg_set_relation_stats, and N pg_set_attribute_stats (which
> > we lump together in one command for the locking benefits and atomic
> > transaction) would fail entirely if one of the set_attributes named a
> > column that we had dropped. It's up for debate whether that's the
> > right behavior or not.
>
> I'd probably make the dropped column a WARNING with a message like
> "skipping dropped column whatever". Regardless, have some kind of
> explanatory comment.
>

That's certainly do-able.




>
> >
> > I pulled most of the hardcoded values from pg_stats itself. The
> > sample set is trivially small, and the values inserted were in-order-
> > ish. So maybe that's why.
>
> In my simple test, most_common_freqs is descending:
>
>    CREATE TABLE a(i int);
>    INSERT INTO a VALUES(1);
>    INSERT INTO a VALUES(2);
>    INSERT INTO a VALUES(2);
>    INSERT INTO a VALUES(3);
>    INSERT INTO a VALUES(3);
>    INSERT INTO a VALUES(3);
>    INSERT INTO a VALUES(4);
>    INSERT INTO a VALUES(4);
>    INSERT INTO a VALUES(4);
>    INSERT INTO a VALUES(4);
>    ANALYZE a;
>    SELECT most_common_vals, most_common_freqs
>      FROM pg_stats WHERE tablename='a';
>     most_common_vals | most_common_freqs
>    ------------------+-------------------
>     {4,3,2}          | {0.4,0.3,0.2}
>    (1 row)
>
> Can you show an example where it's not?
>

Not off hand, no.



>
> >
> > Maybe we could have the functions restricted to a role or roles:
> >
> > 1. pg_write_all_stats (can modify stats on ANY table)
> > 2. pg_write_own_stats (can modify stats on tables owned by user)
>
> If we go that route, we are giving up on the ability for users to
> restore stats on their own tables. Let's just be careful about
> validating data to mitigate this risk.
>

A great many test cases coming in the next patch.