Horton

Re: Statistics Import and Export

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Tom Lane <tgl@sss.pgh.pa.us>, Michael Paquier <michael@paquier.xyz>, jian he <jian.universality@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>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, alvherre@alvh.no-ip.org
Date: 2025-03-06T14:29:07Z
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.

Hi,


On 2025-03-05 23:04:29 -0500, Corey Huinker wrote:
> > > I'm uncertain how we'd do that with (schemaname,tablename) pairs. Are you
> > > suggesting we back the joins from pg_stats to pg_namespace and pg_class
> > and
> > > then filter by oids?
> >
> > I was thinking of one query per schema or something like that. But yea, a
> > query to pg_namespace and pg_class wouldn't be a problem if we did it far
> > fewer times than before.   Or you could put the list of catalogs / tables
> > to
> > be queried into an unnest() with two arrays or such.
> >
> > Not sure how good the query plan for that would be, but it may be worth
> > looking at.
> >
>
> Ok, so we're willing to take the pg_class/pg_namespace join hit for one or
> a handful of queries, good to know.

It's a tradeoff that needs to be evaluated. But I'd be rather surprised if it
weren't faster to run one query with the additional joins than hundreds of
queries without them.


> > > Each call to getAttributeStats() fetches the pg_stats for one and only
> > one
> > > relation and then writes the SQL call to fout, then discards the result
> > set
> > > once all the attributes of the relation are done.
> >
> > I don't think that's true. For one my example demonstrated that it
> > increases
> > the peak memory usage substantially. That'd not be the case if the data was
> > just written out to stdout or such.
> >
> > Looking at the code confirms that. The ArchiveEntry() in
> > dumpRelationStats()
> > is never freed, afaict. And ArchiveEntry() strdups ->createStmt, which
> > contains the "SELECT pg_restore_attribute_stats(...)".
> >
>
> Pardon my inexperience, but aren't the ArchiveEntry records needed right up
> until the program's run?

s/the/the end of the/?


> If there's value in freeing them, why isn't it being done already? What
> other thing would consume this freed memory?

I'm not saying that they can be freed, they can't right now. My point is just
that we *already* keep all the stats in memory, so the fact that fetching all
stats in a single query would also require keeping them in memory is not an
issue.

But TBH, I do wonder how much the current memory usage of the statistics
dump/restore support is going to bite us. In some cases this will dramatically
increase pg_dump/pg_upgrade's memory usage, my tests were with tiny amounts of
data and very simple scalar datatypes and you already could see a substantial
increase.  With something like postgis or even just a lot of jsonb columns
this is going to be way worse.

Greetings,

Andres Freund