Horton

Re: Statistics Import and Export

Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>

View thread
From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2023-11-07T09:23:54Z
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 Tue, Oct 31, 2023 at 12:55 PM Corey Huinker <corey.huinker@gmail.com> wrote:
>>
>>
>> Yeah, that use makes sense as well, and if so then postgres_fdw would likely need to be aware of the appropriate query for several versions back - they change, not by much, but they do change. So now we'd have each query text in three places: a system view, postgres_fdw, and the bin/scripts pre-upgrade program. So I probably should consider the best way to share those in the codebase.
>>
>
> Attached is v2 of this patch.
>
> New features:
> * imports index statistics. This is not strictly accurate: it re-computes index statistics the same as ANALYZE does, which is to say it derives those stats entirely from table column stats, which are imported, so in that sense we're getting index stats without touching the heap.
> * now support extended statistics except for MCV, which is currently serialized as an difficult-to-decompose bytea field.
> * bare-bones CLI script pg_export_stats, which extracts stats on databases back to v12 (tested) and could work back to v10.
> * bare-bones CLI script pg_import_stats, which obviously only works on current devel dbs, but can take exports from older versions.
>

I did a small experiment with your patches. In a separate database
"fdw_dst" I created a table t1 and populated it with 100K rows
#create table t1 (a int, b int);
#insert into t1 select i, i + 1 from generate_series(1, 100000) i;
#analyse t1;

In database "postgres" on the same server, I created a foreign table
pointing to t1
#create server fdw_dst_server foreign data wrapper postgres_fdw
OPTIONS ( dbname 'fdw_dst', port '5432');
#create user mapping for public server fdw_dst_server ;
#create foreign table t1 (a int, b int) server fdw_dst_server;

The estimates are off
#explain select * from t1 where a = 100;
                        QUERY PLAN
-----------------------------------------------------------
 Foreign Scan on t1  (cost=100.00..142.26 rows=13 width=8)
(1 row)

Export and import stats for table t1
$ pg_export_stats -d fdw_dst | pg_import_stats -d postgres

gives accurate estimates
#explain select * from t1 where a = 100;
                        QUERY PLAN
-----------------------------------------------------------
 Foreign Scan on t1  (cost=100.00..1793.02 rows=1 width=8)
(1 row)

In this simple case it's working like a charm.

Then I wanted to replace all ANALYZE commands in postgres_fdw.sql with
import and export of statistics. But I can not do that since it
requires table names to match. Foreign table metadata stores the
mapping between local and remote table as well as column names. Import
can use that mapping to install the statistics appropriately. We may
want to support a command or function in postgres_fdw to import
statistics of all the tables that point to a given foreign server.
That may be some future work based on your current patches.

I have not looked at the code though.

--
Best Wishes,
Ashutosh Bapat