On Fri, Mar 10, 2023 at 4:13 PM Richard Guo <guofengli...@gmail.com> wrote:
> I wonder if we should consider syn_xxxhand rather than min_xxxhand in > clause_is_computable_at when we check if clause mentions any nullable > Vars. But I'm not sure about that. > No, considering syn_xxxhand is not right. After some join order commutation we may form the join with only its min_lefthand and min_righthand. In this case if we check against syn_xxxhand rather than min_xxxhand in clause_is_computable_at, we may end up with being unable to find a proper place for some quals. I can see this problem in below query. select * from t1 left join ((select t2.x from t2 left join t3 on t2.x where t3.x is null) s left join t4 on s.x) on s.x = t1.x; Suppose we've formed join t1/t2 and go ahead to form the join of t1/t2 to t3. If we consider t1/t2 join's syn_xxxhand, then the pushed down qual 't3.x is null' would not be computable at this level because it mentions nullable Vars from t1/t2 join's syn_righthand and meanwhile is not marked with t1/t2 join. This is not correct and would trigger an Assert. Back to the original issue, if a join has more than one quals, actually we treat them as a whole when we check if identity 3 applies as well as when we adjust them to be suitable for commutation according to identity 3. So when we check if a qual is computable at a given level, I think we should also consider the join's quals as a whole. I'm thinking that we use a 'group' notion for RestrictInfos and then use the clause_relids of the 'group' in clause_is_computable_at. Does this make sense? Thanks Richard
