Horton

Re: Statistics Import and Export

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Tom Lane <tgl@sss.pgh.pa.us>, Michael Paquier <michael@paquier.xyz>, jian he <jian.universality@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: 2025-03-06T03:41:24Z
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 2025-03-05 22:00:42 -0500, Corey Huinker wrote:
> On Wed, Mar 5, 2025 at 9:18 PM Andres Freund <andres@anarazel.de> wrote:
> > On 2025-03-05 20:54:35 -0500, Corey Huinker wrote:
> > > It's been considered and not ruled out, with a "let's see how the simple
> > > thing works, first" approach. Considerations are:
> > >
> > > * pg_stats is keyed on schemaname + tablename (which can also be indexes)
> > > and we need to use that because of the security barrier
> >
> > I don't think that has to be a big issue, you can just make the the query
> > query multiple tables at once using an = ANY(ARRAY[]) expression or such.
> >
>
> I'm uncertain how we'd do that with (schemaname,tablename) pairs. Are you
> suggesting we back the joins from pg_stats to pg_namespace and pg_class and
> then filter by oids?

I was thinking of one query per schema or something like that. But yea, a
query to pg_namespace and pg_class wouldn't be a problem if we did it far
fewer times than before.   Or you could put the list of catalogs / tables to
be queried into an unnest() with two arrays or such.

Not sure how good the query plan for that would be, but it may be worth
looking at.


> > > * The stats data is kinda heavy (most common value lists, most common
> > > elements lists, esp for high stattargets), which would be a considerable
> > > memory impact and some of those stats might not even be needed (example,
> > > index stats for a table that is filtered out)
> >
> > Doesn't the code currently have this problem already? Afaict the stats are
> > currently all stored in memory inside pg_dump.
> >
>
> Each call to getAttributeStats() fetches the pg_stats for one and only one
> relation and then writes the SQL call to fout, then discards the result set
> once all the attributes of the relation are done.

I don't think that's true. For one my example demonstrated that it increases
the peak memory usage substantially. That'd not be the case if the data was
just written out to stdout or such.

Looking at the code confirms that. The ArchiveEntry() in dumpRelationStats()
is never freed, afaict. And ArchiveEntry() strdups ->createStmt, which
contains the "SELECT pg_restore_attribute_stats(...)".


> I don't think the query itself would be a problem, a query querying all the
> > required stats should probably use PQsetSingleRowMode() or
> > PQsetChunkedRowsMode().
>
>
> That makes sense if we get the attribute stats from the result set in the
> order that we need them, and I don't know how we could possibly do that.
> We'd still need a table to bsearch() and that would be huge.

I'm not following - what would be the problem with a bsearch()? Compared to
the stats data an array to map from oid to an index in an array of stats data
data would be very small.


But with the unnest() idea from above it wouldn't even be needed, you could
use

SELECT ...
FROM unnest(schema_array, table_array) WITH ORDINALITY AS src(schemaname, tablename)
...
ORDER BY ordinality

or something along those lines.

Greetings,

Andres Freund