I just discovered that queries containing left joins aren't rewritten
when materializations are available and wondered why that is a
limitation. I looked a bit into the implementation and found out that
org.apache.calcite.rel.metadata.RelMdAllPredicates#getAllPredicates(org.apache.calcite.rel.core.Join,
org.apache.calcite.rel.metadata.RelMetadataQuery) returns null when
encountering a non-inner join.
For an inner join, the method returns essentially the join predicate, so
I thought a left join should be similar. How about returning an OR node
that asserts input refs of "outer-joined" tables are null if unmatched?
Let's consider an example query like "from emp e left join dept d on
e.deptno = d.deptno"
If the join were an inner join, the return would be "=(e.deptno,
d.deptno)". In case of a left join I'd suppose it returns
"OR(IS_NULL(d.deptno),=(e.deptno, d.deptno))". Would that be sufficient
to implement left join support or do you have something different in
mind? I couldn't find a ticket for that particular case to discuss a
possible implementation so please direct me if you already tracked that
issue.
--
Mit freundlichen Grüßen,
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*Christian Beikov*
