Re: pg_plan_advice

On Fri, Mar 6, 2026 at 9:47 AM David G. Johnston
<david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> Thank you for putting in the time to respond. That was quite helpful. I've tweaked my tooling to help me remember to do this going forward.

Thanks. Here's v19. I've incorporated a bunch of your changes, some
that were fixes for clear errors and a few of the more stylistic cases
where I decided that I agreed with you, and I've also fixed some
similar things upon which you did not remark. Separately, I've also
committed the patches that were previously 0001, 0002, and 0005, so
all the preparatory stuff is done now, and this version just contains
the substantive commits, for which I am still in need of review,
especially for 0004. I have also made a minor code adjustment:
pg_plan_advice.trace_mask now prints the subplan name except for the
toplevel "subplan".

Here are a few comments on some of the changes you made that I did not adopt:

- In the README, "we" are the PostgreSQL developers and "the user" is
the person using the module. In the documentation, "you" is the user.
If there are deviations from this idea, we should fix them, but it
seems OK that the two documents have different rules, inasmuch as they
address different audiences.

- For the same reason, I chose not to copy the note about
enable_partitionwise_join=off into the README, as that is not a core
concept for developers but a likely error for users. We also don't
really want to start duplicating content too much; arguably, we have
too much of that already.

- I don't really think we need to document the exact rules for
argument to, e.g., the pg_stash_advice functions. I think that makes
the documentation ponderous without adding any real utility. At most,
I would mention in some centralized place what the rules for stash
names are. Separately, there is the question of whether the current
naming rules are the right ones.

- In a lot of places, especially in the README, I just disagreed with
your choice of what to emphasize. For instance, I thought my longer
explanation of how we must not infer guidance from the absence of
advice was better than your shorter one, because it's such a critical
point for future developers touching this code to understand. On the
other hand, saying that advice should be stable in the face of
statistics changes was redundant: if it doesn't even do that much,
then what would even be the point?

- I chose to retain the use of the term "core planner" in the SGML
documentation rather than your suggestion of deleting the word "core".
I do not love the wording I've chosen here, but I don't love your
change, either. It seems to me that there is a risk of people being
confused about the distinction between the planning-related logic in
src/backend/optimizer and the planning-related object in
pg_plan_advice itself. I included the word in core for emphasis.
Whether this is the right idea is debatable, of course.

Thanks,

--
Robert Haas
EDB: http://www.enterprisedb.com