Horton

Re: Statistics Import and Export

Stephen Frost <sfrost@snowman.net>

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From: Stephen Frost <sfrost@snowman.net>
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Corey Huinker <corey.huinker@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>
Date: 2024-03-06T10:33:02Z
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.

Greetings,

On Wed, Mar 6, 2024 at 11:07 Matthias van de Meent <
boekewurm+postgres@gmail.com> wrote:

> On Fri, 1 Mar 2024, 04:55 Corey Huinker, <corey.huinker@gmail.com> wrote:
> >> Also per our prior discussion- this makes sense to include in post-data
> section, imv, and also because then we have the indexes we may wish to load
> stats for, but further that also means it’ll be in the paralleliziable part
> of the process, making me a bit less concerned overall about the individual
> timing.
> >
> >
> > The ability to parallelize is pretty persuasive. But is that
> per-statement parallelization or do we get transaction blocks? i.e. if we
> ended up importing stats like this:
> >
> > BEGIN;
> > LOCK TABLE schema.relation IN SHARE UPDATE EXCLUSIVE MODE;
> > LOCK TABLE pg_catalog.pg_statistic IN ROW UPDATE EXCLUSIVE MODE;
> > SELECT pg_import_rel_stats('schema.relation', ntuples, npages);
> > SELECT pg_import_pg_statistic('schema.relation', 'id', ...);
> > SELECT pg_import_pg_statistic('schema.relation', 'name', ...);
>
> How well would this simplify to the following:
>
> SELECT pg_import_statistic('schema.relation', attname, ...)
> FROM (VALUES ('id', ...), ...) AS relation_stats (attname, ...);


Using a VALUES construct for this does seem like it might make it cleaner,
so +1 for investigating that idea.

Or even just one VALUES for the whole statistics loading?


I don’t think we’d want to go beyond one relation at a time as then it can
be parallelized, we won’t be trying to lock a whole bunch of objects at
once, and any failures would only impact that one relation’s stats load.

I suspect the main issue with combining this into one statement
> (transaction) is that failure to load one column's statistics implies
> you'll have to redo all the other statistics (or fail to load the
> statistics at all), which may be problematic at the scale of thousands
> of relations with tens of columns each.


I’m pretty skeptical that “stats fail to load and lead to a failed
transaction” is a likely scenario that we have to spend a lot of effort
on.  I’m pretty bullish on the idea that this simply won’t happen except in
very exceptional cases under a pg_upgrade (where the pg_dump that’s used
must match the target server version) and where it happens under a pg_dump
it’ll be because it’s an older pg_dump’s output and the answer will likely
need to be “you’re using a pg_dump file generated using an older version of
pg_dump and need to exclude stats entirely from the load and instead run
analyze on the data after loading it.”

What are the cases where we would be seeing stats reloads failing where it
would make sense to re-try on a subset of columns, or just generally, if we
know that the pg_dump version matches the target server version?

Thanks!

Stephen

>