Horton

Re: Statistics Import and Export

Corey Huinker <corey.huinker@gmail.com>

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From: Corey Huinker <corey.huinker@gmail.com>
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: Jeff Davis <pgsql@j-davis.com>, Stephen Frost <sfrost@snowman.net>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>
Date: 2024-03-27T06:20:41Z
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.

>
>
>
> +\gexec
>
> Why do we need to construct the command and execute? Can we instead
> execute the function directly? That would also avoid ECHO magic.
>

We don't strictly need it, but I've found the set-difference operation to
be incredibly useful in diagnosing problems. Additionally, the values are
subject to change due to changes in test data, no guarantee that the output
of ANALYZE is deterministic, etc. But most of all, because the test cares
about the correct copying of values, not the values themselves.


>
> +   <table id="functions-admin-statsimport">
> +    <title>Database Object Statistics Import Functions</title>
> +    <tgroup cols="1">
> +     <thead>
> +      <row>
> +       <entry role="func_table_entry"><para role="func_signature">
> +        Function
> +       </para>
> +       <para>
> +        Description
> +       </para></entry>
> +      </row>
> +     </thead>
>
> COMMENT: The functions throw many validation errors. Do we want to list
> the acceptable/unacceptable input values in the documentation corresponding
> to those? I don't expect one line per argument validation. Something like
> "these, these and these arguments can not be NULL" or "both arguments in
> each of the pairs x and y, a and b, and c and d should be non-NULL or NULL
> respectively".
>

Yes. It should.


>  Statistics are about data. Whenever pg_dump dumps some filtered data, the
> statistics collected for the whole table are uselss. We should avoide
> dumping
> statistics in such a case. E.g. when only schema is dumped what good is
> statistics? Similarly the statistics on a partitioned table may not be
> useful
> if some its partitions are not dumped. Said that dumping statistics on
> foreign
> table makes sense since they do not contain data but the statistics still
> makes sense.
>

Good points, but I'm not immediately sure how to enforce those rules.


>
>
>>
>> Key areas where I'm seeking feedback:
>>
>> - What level of errors in a restore will a user tolerate, and what should
>> be done to the error messages to indicate that the data itself is fine, but
>> a manual operation to update stats on that particular table is now
>> warranted?
>> - To what degree could pg_restore/pg_upgrade take that recovery action
>> automatically?
>> - Should the individual attribute/class set function calls be grouped by
>> relation, so that they all succeed/fail together, or should they be called
>> separately, each able to succeed or fail on their own?
>> - Any other concerns about how to best use these new functions.
>>
>>
>>
> Whether or not I pass --no-statistics, there is no difference in the dump
> output. Am I missing something?
> $ pg_dump -d postgres > /tmp/dump_no_arguments.out
> $ pg_dump -d postgres --no-statistics > /tmp/dump_no_statistics.out
> $ diff /tmp/dump_no_arguments.out /tmp/dump_no_statistics.out
> $
>
> IIUC, pg_dump includes statistics by default. That means all our pg_dump
> related tests will have statistics output by default. That's good since the
> functionality will always be tested. 1. We need additional tests to ensure
> that the statistics is installed after restore. 2. Some of those tests
> compare dumps before and after restore. In case the statistics is changed
> because of auto-analyze happening post-restore, these tests will fail.
>

+1


> I believe, in order to import statistics through IMPORT FOREIGN SCHEMA,
> postgresImportForeignSchema() will need to add SELECT commands invoking
> pg_set_relation_stats() on each imported table and pg_set_attribute_stats()
> on each of its attribute. Am I right? Do we want to make that happen in the
> first cut of the feature? How do you expect these functions to be used to
> update statistics of foreign tables?
>

I don't think there's time to get it into this release. I think we'd want
to extend this functionality to both IMPORT FOREIGN SCHEMA and ANALYZE for
foreign tables, in both cases with a server/table option to do regular
remote sampling. In both cases, they'd do a remote query very similar to
what pg_dump does (hence putting it in fe_utils), with some filters on
which columns/tables it believes it can trust. The remote table might
itself be a view (in which case they query would turn up nothing) or column
data types may change across the wire, and in those cases we'd have to fall
back to sampling.