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Re: pg_plan_advice

Lukas Fittl <lukas@fittl.com>

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From: Lukas Fittl <lukas@fittl.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jakub Wartak <jakub.wartak@enterprisedb.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-04-04T08:11:25Z
Lists: pgsql-hackers 
On Thu, Apr 2, 2026 at 7:15 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Thu, Apr 2, 2026 at 12:15 PM Robert Haas <robertmhaas@gmail.com> wrote:
> > So here's v24, also dropping pg_collect_advice.
>
> That version didn't actually pass CI. Here's v25.

I've reviewed the pg_stash_advice code and documentation, and I think
this is overall sound.  As I mentioned previously, I think its a very
important addition to make pg_plan_advice work for practical problems
end users encounter.

To me this looks good to go, with three minor notes below. For context
I've spent a few hours today going through the code manually, and
doing testing.

And thank you for the detailed notes in your earlier email, and
reworking this. Regarding authorship, I'm happy to be listed as
co-author on the persistence part if you want to keep that in the
commit message. Overall I'm also willing to put in work during the
remaining cycle to test/review/address issues in pg_plan_advice or
pg_stash_advice, so we can hopefully sort out the other items being
discussed.

For 0001:

> diff --git a/doc/src/sgml/pgstashadvice.sgml b/doc/src/sgml/pgstashadvice.sgml
> new file mode 100644
> index 00000000000..ec60552a447
> --- /dev/null
> +++ b/doc/src/sgml/pgstashadvice.sgml
> @@ -0,0 +1,216 @@
> ...
> +   <varlistentry>
> +    <term>
> +     <function>pg_create_advice_stash(stash_name text) returns void</function>
> +     <indexterm>
> +      <primary>pg_create_advice_stash</primary>
> +     </indexterm>
> +    </term>
> +
> +    <listitem>
> +     <para>
> +      Creates a new, empty advice stash with the given name.
> +     </para>
> +    </listitem>
> +   </varlistentry>
> +

I think we should document the restrictions on advice names here (i.e.
they must be alphanumeric or contain an underscore, not start with a
digit, and maximum NAMEDATLEN).

For 0002:

> diff --git a/contrib/pg_stash_advice/pg_stash_advice.c b/contrib/pg_stash_advice/pg_stash_advice.c
> index 15e7adf849b..1858c6a135a 100644
> --- a/contrib/pg_stash_advice/pg_stash_advice.c
> +++ b/contrib/pg_stash_advice/pg_stash_advice.c
> ...
> @@ -464,6 +522,43 @@ pgsa_drop_stash(char *stash_name)
>         }
>     }
>     dshash_seq_term(&iterator);
> +
> +    /* Bump change count. */
> +    pg_atomic_add_fetch_u64(&pgsa_state->change_count, 1);
> +}
> +
> +/*
> + * Remove all stashes and entries from shared memory.
> + *
> + * This is intended to be called before reloading from a dump file, so that
> + * a failed previous attempt doesn't leave stale data behind.
> + */
> +void
> +pgsa_reset_all_stashes(void)
> +{

I think this might be good to expose on the SQL level as well - in
case someone accidentally created a lot of stashes it could be tedious
to remove them all, e.g. if they wanted to clear all the memory after
an experiment.

> diff --git a/contrib/pg_stash_advice/stashpersist.c b/contrib/pg_stash_advice/stashpersist.c
> new file mode 100644
> index 00000000000..da96ee0d803
> --- /dev/null
> +++ b/contrib/pg_stash_advice/stashpersist.c
>...
> +            /* Parse the query ID. */
> +            errno = 0;
> +            queryId = strtoll(queryid_str, &endptr, 10);
> +            if (*endptr != '\0' || errno != 0 || queryid_str == endptr)
> +                ereport(ERROR,
> +                        (errcode(ERRCODE_DATA_CORRUPTED),
> +                         errmsg("syntax error in file \"%s\" line %u: invalid query ID \"%s\"",
> +                                PGSA_DUMP_FILE, lineno, queryid_str)));
> +

It might be worth adding a queryId == 0 check here, since we won't
check it later, and its helpful to avoid unpredictable behavior just
in case someone decided to mess with the file manually.

Thanks,
Lukas

-- 
Lukas Fittl

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_plan_advice: Fix another unique-semijoin bug.

  2. pg_plan_advice: Export feedback-related definitions.

  3. pg_plan_advice: Fix a bug when a subquery is pruned away entirely.

  4. pg_plan_advice: Add alternatives test to Makefile.

  5. pg_plan_advice: Handle non-repeatable TABLESAMPLE scans.

  6. pg_stash_advice: Allow stashed advice to be persisted to disk.

  7. Add pg_stash_advice contrib module.

  8. pg_plan_advice: Avoid assertion failure with partitionwise aggregate.

  9. pg_plan_advice: Invent DO_NOT_SCAN(relation_identifier).

  10. Add an alternative_plan_name field to PlannerInfo.

  11. pg_plan_advice: Refactor to invent pgpa_planner_info

  12. Respect disabled_nodes in fix_alternative_subplan.

  13. get_memoize_path: Don't exit quickly when PGS_NESTLOOP_PLAIN is unset.

  14. test_plan_advice: Set TAP test priority 50 in meson.build.

  15. pg_plan_advice: Avoid a crash under GEQO.

  16. Test pg_plan_advice using a new test_plan_advice module.

  17. pg_plan_advice: Always install pg_plan_advice.h, and in the right place

  18. pg_plan_advice: Fix failures to accept identifier keywords.

  19. Add pg_plan_advice contrib module.

  20. Allow extensions to mark an individual index as disabled.

  21. Replace get_relation_info_hook with build_simple_rel_hook.

  22. Store information about Append node consolidation in the final plan.

  23. Store information about elided nodes in the final plan.

  24. Store information about range-table flattening in the final plan.

  25. Pass cursorOptions to planner_setup_hook.

  26. Fix PGS_CONSIDER_NONPARTIAL interaction with Materialize nodes.

  27. Fix mistakes in commit 4020b370f214315b8c10430301898ac21658143f

  28. Allow for plugin control over path generation strategies.

  29. Update some comments for fasthash

  30. Allow passing a pointer to GetNamedDSMSegment()'s init callback.

  31. Don't reset the pathlist of partitioned joinrels.

  32. Treat number of disabled nodes in a path as a separate cost metric.