Horton

Re: Statistics Import and Export

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2023-11-02T13:52:20Z
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.

On 11/2/23 06:01, Corey Huinker wrote:
> 
> 
>     Maybe I just don't understand, but I'm pretty sure ANALYZE does not
>     derive index stats from column stats. It actually builds them from the
>     row sample.
> 
> 
> That is correct, my error.
>  
> 
> 
>     > * now support extended statistics except for MCV, which is currently
>     > serialized as an difficult-to-decompose bytea field.
> 
>     Doesn't pg_mcv_list_items() already do all the heavy work?
> 
> 
> Thanks! I'll look into that.
> 
> The comment below in mcv.c made me think there was no easy way to get
> output.
> 
> /*
>  * pg_mcv_list_out      - output routine for type pg_mcv_list.
>  *
>  * MCV lists are serialized into a bytea value, so we simply call byteaout()
>  * to serialize the value into text. But it'd be nice to serialize that into
>  * a meaningful representation (e.g. for inspection by people).
>  *
>  * XXX This should probably return something meaningful, similar to what
>  * pg_dependencies_out does. Not sure how to deal with the deduplicated
>  * values, though - do we want to expand that or not?
>  */
> 

Yeah, that was the simplest output function possible, it didn't seem
worth it to implement something more advanced. pg_mcv_list_items() is
more convenient for most needs, but it's quite far from the on-disk
representation.

That's actually a good question - how closely should the exported data
be to the on-disk format? I'd say we should keep it abstract, not tied
to the details of the on-disk format (which might easily change between
versions).

I'm a bit confused about the JSON schema used in pg_statistic_export
view, though. It simply serializes stakinds, stavalues, stanumbers into
arrays ... which works, but why not to use the JSON nesting? I mean,
there could be a nested document for histogram, MCV, ... with just the
correct fields.

  {
    ...
    histogram : { stavalues: [...] },
    mcv : { stavalues: [...], stanumbers: [...] },
    ...
  }

and so on. Also, what does TRIVIAL stand for?

regards

-- 
Tomas Vondra
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