Github user yhuai commented on the pull request:
https://github.com/apache/spark/pull/9055#issuecomment-149777477
Two general comments. First, we need to add document to explain how we
rewrite a plan when (1) there is a uncorrelated subquery and (2) there is a
correlated subquery. Second, for those rewriting rules, I am thinking if we can
have more concise ones. For uncorrelated subqueries, the subquery itself should
be a resolved logical plan, right? For correlated subqueries, we only need to
extract those conditions referring columns in the outer query block, right? Do
we really need to matching those different specific patterns? Can we have some
general logics?
Actually, does this pr try to support uncorrelated in/not in/exists/not
exists subqueries?
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