Horton

Re: Statistics Import and Export

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jeff Davis <pgsql@j-davis.com>, Corey Huinker <corey.huinker@gmail.com>, Michael Paquier <michael@paquier.xyz>, jian he <jian.universality@gmail.com>, 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>, 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: 2025-02-21T21:11:48Z
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 2025-02-21 15:49:10 -0500, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > Which to me rather strongly suggests pg_dump has gotten a *lot* slower with
> > this change.
>
> Well, it's doing strictly more work, so somewhat slower is to be
> expected.

Yea, if we had talked a few percent, I'd not have balked.  It's more like 2-4x
though and it'll probably be worse when not connecting over local TCP
connections.

This is a slowdown to the point that the downtime for pg_upgrade will be
substantially lengthened compared to before.  But I think we should be able to
address that to a large degree.


> In a quick look at the committed patch, it doesn't seem to have
> used any of the speedup strategies we applied to pg_dump a couple
> of years ago.  One or the other of these should help:
>
> * Issue a single query to fetch stats from every table we're dumping
> * Set up a prepared query to avoid re-planning the per-table query
>   (compare be85727a3)
>
> I'm not sure how workable the first of these would be though.
> It's not hard to imagine it blowing out pg_dump's memory usage
> for a DB with a lot of tables and high default_statistics_target.

We could presumably do the one-query approach for the relation stats, that's
just three integers.  That way we'd at least not end up with two queries for
each table (for pg_class.reltuples etc and for pg_stats).

I guess the memory usage could also be addressed by using COPY, but that's
probably unrealistically complicated.


> The second one should be relatively downside-free.

Yea. And at least with pg_dump running locally that's where a lot of the CPU
time is spent.

Remotely doing lots of one-by-one queries will hurt even with prepared
statements though.

One way to largely address that would be to use a prepared statement combined
with libpq pipelining.  That still has separate executor startup etc, but I
think it should still reduce the cost to a point where we don't care anymore.

Greetings,

Andres Freund