Re: Memory consumed by child SpecialJoinInfo in partitionwise join planning

On Wed, Sep 27, 2023 at 2:30 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
>
> Here are some comments.

Thanks for your review.

>
> Please merge 0003 into 0002.

Done.

>
> + /*
> + * But the list of operator OIDs and the list of expressions may be
> + * referenced somewhere else. Do not free those.
> + */
>
> I don't see build_child_join_sjinfo() copying any list of OIDs, so I'm
> not sure what the comment is referring to. Also, instead of "the list
> of expressions", it might be more useful to mention semi_rhs_expr,
> because that's the only one being copied.

list of OID is semi_operators. It's copied as is from parent
SpecialJoinInfo. But the way it's mentioned in the comment is
confusing. I have modified the prologue of function to provide a
general guidance on what can be freed and what should not be. and then
specific examples. Let me know if this is more clear.

>
> The comment for SpecialJoinInfo mentions that they are stored in
> PlannerInfo.join_info_list, but the child SpecialJoinInfos used by
> partitionwise joins are not, which I noticed has not been mentioned
> anywhere. Should we make a note of that in the SpecialJoinInfo's
> comment?

Good point. Since SpecialJoinInfos for JOIN_INNER are mentioned there,
it makes sense to mention transient child SpecialJoinInfos as well.
Done.

>
> Just out of curiosity, is their not being present in join_info_list
> problematic in some manner, such as missed optimization opportunities
> for child joins? I noticed there is a loop over join_info_list in
> add_paths_to_join_rel(), which does get called for child joinrels. I
> know this a bit off-topic for the thread, but thought to ask in case
> you've already thought about it.

The function has a comment and code to take care of this at the very beginning
/*
* PlannerInfo doesn't contain the SpecialJoinInfos created for joins
* between child relations, even if there is a SpecialJoinInfo node for
* the join between the topmost parents. So, while calculating Relids set
* representing the restriction, consider relids of topmost parent of
* partitions.
*/
if (joinrel->reloptkind == RELOPT_OTHER_JOINREL)
joinrelids = joinrel->top_parent_relids;
else
joinrelids = joinrel->relids;

PFA latest patch set
0001 - same as previous patch set
0002 - 0002 and 0003 from previous patch set squashed together
0003 - changes addressing above comments.

--
Best Wishes,
Ashutosh Bapat