Horton

Re: Statistics Import and Export

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
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: 2025-02-21T21:24:38Z
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.

Andres Freund <andres@anarazel.de> writes:
> 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

Oy.  Those are outright horrid, even without any consideration of
pre-preparing them.  We know the OID of the table we want to dump,
we should be doing "FROM pg_class WHERE oid = whatever" and lose
the join to pg_namespace altogether.  The explicit casts to regclass
are quite expensive too to fetch information that pg_dump already
has.  It already knows the server version, too.

Moreover, the first of these shouldn't be a separate query at all.
I objected to fetching pg_statistic content for all tables at once,
but relpages/reltuples/relallvisible is a pretty small amount of
new info.  We should just collect those fields as part of getTables'
main query of pg_class (which, indeed, is already fetching relpages).

On the second one, if we want to go through the pg_stats view then
we can't rely on table OID, but I don't see why we need the joins
to anything else.  "WHERE s.schemaname = 'x' AND s.tablename = 'y'"
seems sufficient.

I wonder whether we ought to issue different queries depending on
whether we're superuser.  The pg_stats view is rather expensive
because of its security restrictions, and if we're superuser we
could just look directly at pg_statistic.  Maybe those checks are
fast enough not to matter, but ...

			regards, tom lane