Re: Removing unneeded self joins

On Mon, May 6, 2024 at 12:24:41PM -0400, Robert Haas wrote:
> Now that being said, I do also agree that the planner code is quite
> hard to understand, for various reasons. I don't think the structure
> of that code and the assumptions underlying it are as well-documented
> as they could be, and neither do I think that all of them are optimal.
> It has taken me a long time to learn as much as I know, and there is
> still quite a lot that I don't know. And I also agree that the planner
> does an unfortunate amount of in-place modification of existing
> structures without a lot of clarity about how it all works, and an
> unfortunate amount of data copying in some places, and even that the
> partition-wise join code isn't all that it could be. But I do not
> think that adds up to a conclusion that we should just be less
> ambitious with planner changes. Indeed, I would like to see us do
> more. There is certainly a lot of useful work that could be done. The
> trick is figuring out how to do it without breaking too many things,
> and that is not easy.

I agree with Robert. While writting the Postgres 17 release notes, I am
excited to see the many optimizer improvements, and removing self-joins
from that list will be unfortunate.

I did write a blog entry in 2021 that suggested we could have
optimizer aggressiveness control to allow for more expensive
optimizations:

https://momjian.us/main/blogs/pgblog/2021.html#May_14_2021

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

Only you can decide what is important to you.