Re: PATCH: generate fractional cheapest paths in generate_orderedappend_path

Hi,

thanks for the quick reply!

From: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
Sent: Thursday, June 3, 2021 20:11
To: Arne Roland; pgsql-hackers
Subject: Re: PATCH: generate fractional cheapest paths in generate_orderedappend_path

> I haven't tested the parallel case, but I think we should sort out (3)
> get_cheapest_fractional_path_for_pathkeys as mentioned above.
>

Not sure what you refer to by "above" - it's probably better to reply
in-line to existing message, which makes it much cleared.

I was referring to one message above. I thought the thread was still short enough. Apparently to much time has passed. Sorry, I hope this mail is better. I was referring to my post from April:
________________________________
From: Arne Roland
Sent: Monday, April 26, 2021 13:00
To: Tomas Vondra; pgsql-hackers
Subject: Re: PATCH: generate fractional cheapest paths in generate_orderedappend_path

>3) Not sure if get_cheapest_fractional_path_for_pathkeys should worry

> about require_parallel_safe, just like the other functions nearby.

I think it should. We have a ParallelAppend node after all.
I'm not really familiar with the way get_cheapest_fractional_path_for_pathkeys is used, but a quick search suggests to me, that build_minmax_path was thus far the only one using it. And minmax paths are never parallel safe anyway. I think that is the reason it doesn't do that already.

________________________________
From: Zhihong Yu <zyu(at)yugabyte(dot)com>
Sent: Thursday, June 3, 2021 22:50
To: Tomas Vondra
Cc: Arne Roland; pgsql-hackers
Subject: Re: PATCH: generate fractional cheapest paths in generate_orderedappend_path

Hi,
In REL_11_STABLE branch, a search revealed the following:

src/backend/optimizer/path/pathkeys.c: * get_cheapest_fractional_path_for_pathkeys
src/backend/optimizer/path/pathkeys.c:get_cheapest_fractional_path_for_pathkeys(List *paths,
src/backend/optimizer/plan/planagg.c: get_cheapest_fractional_path_for_pathkeys(final_rel->pathlist,
src/include/optimizer/paths.h:extern Path *get_cheapest_fractional_path_for_pathkeys(List *paths,

It seems this function has been refactored out in subsequent releases.

FYI

Thanks for the info!
I doubt there is any interest to back patch this anywhere. My most ambitious dream would be getting this into pg 14.

I think, we only care about a parallel safety aware variant anyways, which afaict never existed.

________________________________
From: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
Sent: Thursday, June 3, 2021 22:57
To: Zhihong Yu
Cc: Arne Roland; pgsql-hackers
Subject: Re: PATCH: generate fractional cheapest paths in generate_orderedappend_path
Actually, there are two comments

/* XXX maybe we should have startup_new_fractional? */

in the patch I posted - I completely forgot about that. But I think
that's a typo, I think - it should be

/* XXX maybe we should have startup_neq_fractional? */

and the new flag would work similarly to startup_neq_total, i.e. it's
pointless to add paths where startup == fractional cost.

At least I think that was the idea when I wrote the patch, it way too
long ago.

Sorry, I almost forgot about this myself. I only got reminded upon seeing that again with different queries/tables.
Just to be sure I get this correctly: You mean startup_gt_fractional (cost) as an additional condition, right?

Regards
Arne