Horton

Re: Statistics Import and Export

Matthias van de Meent <boekewurm+postgres@gmail.com>

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From: Matthias van de Meent <boekewurm+postgres@gmail.com>
To: Stephen Frost <sfrost@snowman.net>
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: 2024-03-06T11:06:28Z
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 Wed, 6 Mar 2024 at 11:33, Stephen Frost <sfrost@snowman.net> wrote:
> On Wed, Mar 6, 2024 at 11:07 Matthias van de Meent <boekewurm+postgres@gmail.com> wrote:
>> 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.

That also makes sense.

>> 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.

Agreed on the "don't have to spend a lot of time on it", but I'm not
so sure on the "unlikely" part while the autovacuum deamon is
involved, specifically for non-upgrade pg_restore. I imagine (haven't
checked) that autoanalyze is disabled during pg_upgrade, but
pg_restore doesn't do that, while it would have to be able to restore
statistics of a table if it is included in the dump (and the version
matches).

> 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?

Last time I checked, pg_restore's default is to load data on a
row-by-row basis without --single-transaction or --exit-on-error. Of
course, pg_upgrade uses it's own set of flags, but if a user is
restoring stats with  pg_restore, I suspect they'd rather have some
column's stats loaded than no stats at all; so I would assume this
requires one separate pg_import_pg_statistic()-transaction for every
column.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)