Optimize Project Operator in Logical Plan

[penghuo]

Because the top projection should always refer the attributes refer in the bottom projection, the adjacent Projection operator could be merged into one Projection.

• Project — Project ----> Project

e.g. Project(a, b) — Project(a, b, c) — AnyOtherOperator ----> Project(a, b) — AnyOtherOperator
When considering the complex cases Project(A/B as C, A*B as D) — Project(a as A, a/b as B), there are two approach to think the problems,

1. Merge to the single one, Project(a/(a/b) as C, a * (a/b) as D).
2. Keep the original Project, only push down the attribute select which related to original doc. The result should be Project(A/B as C, A*B as D) — Project(a as A, a/b as B) — Project(a, b) — AnyOtherOperator

Note: with our current solution, we can not differencated which the ReferenceExpression refer the attribute of the original doc or refer the attribute of the intermediate doc.