Major Description
Currently the children field of a subquery expression is used to store both collected outer references inside the subquery plan, and also join conditions after correlated predicates are pulled up. For example
SELECT (SELECT max(c1) FROM t1 WHERE t1.c1 = t2.c1) FROM t2
After analysis phase:
scalar-subquery [t2.c1]
After PullUpCorrelatedPredicates:
scalar-subquery [t1.c1 = t2.c1]
The references for a subquery expressions is also confusing:
override lazy val references: AttributeSet =
if (plan.resolved) super.references – plan.outputSet else super.references
We should split this children field into outer attribute references and join conditions.
