Horton

Re: Statistics Import and Export

Stephen Frost <sfrost@snowman.net>

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From: Stephen Frost <sfrost@snowman.net>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, pgsql-hackers@lists.postgresql.org
Date: 2024-02-29T23:17:14Z
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.

Greetings,

On Thu, Feb 29, 2024 at 17:48 Corey Huinker <corey.huinker@gmail.com> wrote:

> Having looked through this thread and discussed a bit with Corey
>> off-line, the approach that Tom laid out up-thread seems like it would
>> make the most sense overall- that is, eliminate the JSON bits and the
>> SPI and instead export the stats data by running queries from the new
>> version of pg_dump/server (in the FDW case) against the old server
>> with the intelligence of how to transform the data into the format
>> needed for the current pg_dump/server to accept, through function calls
>> where the function calls generally map up to the rows/information being
>> updated- a call to update the information in pg_class for each relation
>> and then a call for each attribute to update the information in
>> pg_statistic.
>>
>
> Thanks for the excellent summary of our conversation, though I do add that
> we discussed a problem with per-attribute functions: each function would be
> acquiring locks on both the relation (so it doesn't go away) and
> pg_statistic, and that lock thrashing would add up. Whether that overhead
> is judged significant or not is up for discussion. If it is significant, it
> makes sense to package up all the attributes into one call, passing in an
> array of some new pg_statistic-esque special type....the very issue that
> sent me down the JSON path.
>
> I certainly see the flexibility in having a per-attribute functions, but
> am concerned about non-binary-upgrade situations where the attnums won't
> line up, and if we're passing them by name then the function has dig around
> looking for the right matching attnum, and that's overhead too. In the
> whole-table approach, we just iterate over the attributes that exist, and
> find the matching parameter row.
>

That’s certainly a fair point and my initial reaction (which could
certainly be wrong) is that it’s unlikely to be an issue- but also, if you
feel you could make it work with an array and passing all the attribute
info in with one call, which I suspect would be possible but just a bit
more complex to build, then sure, go for it. If it ends up being overly
unwieldy then perhaps the  per-attribute call would be better and we could
perhaps acquire the lock before the function calls..?  Doing a check to see
if we have already locked it would be cheaper than trying to acquire a new
lock, I’m fairly sure.

Also per our prior discussion- this makes sense to include in post-data
section, imv, and also because then we have the indexes we may wish to load
stats for, but further that also means it’ll be in the paralleliziable part
of the process, making me a bit less concerned overall about the individual
timing.

Thanks!

Stephen

>