Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/2405#issuecomment-67432312
Hi, @marmbrus ,the key point why I want to introduce `UnResolvedGetField`
is that: for something like `a.b[0].c.d`, we first parse it to
`GetField(GetField(GetItem(Unresolved("a.b"), 0), "c"), "d")`. Then in
`LogicalPlan#resolve`, we resolve `"a.b"` and build a `GetField` chain from
bottom(the relation). But for the 2 outer `GetFiled`, we have to resolve them
in `Analyzer`, check data type of child, search needed field, etc. which is
similar to what we have done in `LogicalPlan#resolve`.
At first, everything is good as `GetField` is quite simple. But when
`GetFiled` get more complex, such as add resolver logic, support array type,
etc. there will be more and more duplicated code in `LogicalPlan#resolve` and
`Analyzer` and `GetField`.
For now, the searching field logic is duplicated in
`LogicalPlan#resolveNesting` and `GetField#field`, but **not consistent**. In
`resolveNesting`, we check ambiguous reference using `fields.filter`; in
`GetField#field`, we are not.
So I think we need extract the logic of resolving `GetField` to prevent
further trouble.
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