Horton

Re: Statistics Import and Export

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jeff Davis <pgsql@j-davis.com>
Cc: 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>, Tom Lane <tgl@sss.pgh.pa.us>, 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: 2025-02-21T20:57:20Z
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 15:23:00 -0500, Andres Freund wrote:
> On 2025-02-20 01:39:34 -0800, Jeff Davis wrote:
> > Committed with some revisions on top of v48:
>
> This made the pg_upgrade tests considerably slower.
>
> In an assert build without optimization (since that's what I use for normal
> dev work):
>
> 1fd1bd87101^     65.03s
> 1fd1bd87101      86.84s
>
>
> Looking at the times in the in the regress_log, I see:
> [...]
> Which to me rather strongly suggests pg_dump has gotten a *lot* slower with
> this change.

Indeed. While the slowdown is worse with assertions and without compiler
optimizations, it's pretty bad otherwise too.

optimized, non-cassert, pg_dump and server with the regression database contents:

$ time ./src/bin/pg_dump/pg_dump regression > /dev/null

real	0m1.314s
user	0m0.189s
sys	0m0.059s

$ time ./src/bin/pg_dump/pg_dump --no-statistics regression > /dev/null

real	0m0.472s
user	0m0.179s
sys	0m0.035s


Unoptimized, cassert server and pg_dump:

$ time ./src/bin/pg_dump/pg_dump regression > /dev/null

real	0m9.008s
user	0m0.396s
sys	0m0.108s


$ time ./src/bin/pg_dump/pg_dump --no-statistics regression > /dev/null

real	0m2.590s
user	0m0.347s
sys	0m0.037s


Looking at the query log, the biggest culprit is a *lot* of additional
queries, I think primarily these two:


SELECT c.oid::regclass AS relation, current_setting('server_version_num') AS version, c.relpages, c.reltuples, c.relallvisible FROM pg_class c JOIN pg_namespace n ON n.oid = c.relnamespace WHERE n.nspname = 'public' AND c.relname = 'alpha_neg_p2'

SELECT c.oid::regclass AS relation, s.attname,s.inherited,current_setting('server_version_num') AS version, s.null_frac,s.avg_width,s.n_distinct,s.most_common_vals,s.most_common_freqs,s.histogram_bounds,s.correlation,s.most_common_elems,s.most_common_elem_freqs,s.elem_count_histogram,s.range_length_histogram,s.range_empty_frac,s.range_bounds_histogram FROM pg_stats s JOIN pg_namespace n ON n.nspname = s.schemaname JOIN pg_class c ON c.relname = s.tablename AND c.relnamespace = n.oid WHERE s.schemaname = 'public' AND s.tablename = 'alpha_neg_p2' ORDER BY s.attname, s.inherited


I think there are a few things wrong here:

1) Why do we need to plan this over and over? Tom a while ago put in a fair
   bit of work to make frequent queries use prepared statements.

   In this case we spend more time replanning the query than executing it.

2) Querying this one-by-one makes this much more expensive than if it were
   queried in a batched fashion, for multiple tables at once.  This is
   especially true if actually executed over network, rather than locally.

3) The query is unnecessarily expensive due to repeated joins gathering the
   same information.  pg_stats has a join to pg_namespace and pg_class, but
   then the query above joins to both *again*.

   And afaict the joins in the pg_stats query are pretty useless? Isn't all
   that information already available in pg_stats? I guess you did that to get
   it as a ::regclass, but isn't that already known, why requery it?


4) Why do we need to fetch the version twice for every table, that can't be
   right? It won't change while pg_dump is running.


Greetings,

Andres Freund