Re: Assert failure of the cross-check for nullingrels

 

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