Dean Rasheed <dean.a.rasheed@gmail.com> writes:
> On Fri, 27 Sept 2024 at 13:52, Richard Guo <guofenglinux@gmail.com> wrote:
>> I think we can check the jointype of the join between the target and
>> the source relation when adding the vars in mergeJoinCondition. If it
>> is JOIN_LEFT, we mark the vars that belong to source as nullable by
>> this join.
> Another option is to do it in transform_MERGE_to_join(). That feels
> safer, because the jointree might have been modified by the time we
> reach preprocess_targetlist().
Yeah, I think it's critical that these Vars be already correctly
marked before we engage in all the slicing-and-dicing that
prepjointree et al will do. As an example, it seems not impossible
for join removal to make wrong decisions if they aren't.
> Something like the attached.
Could use some comments ... but actually, now I'm confused about why
any of this is the right thing at all:
+ * Similarly, any non-target Vars in the join condition will be added to
+ * the targetlist by preprocess_targetlist(), and so must be marked as
+ * nullable by the join, for LEFT and FULL joins.
Why do we need these Vars in the tlist? If they're for re-evaluating
the join condition, isn't the already-nulled form of them the wrong
thing?
regards, tom lane
