Github user marmbrus commented on the pull request:
https://github.com/apache/spark/pull/2345#issuecomment-55178764
Hey @koeninger, thanks for implementing this optimization!
Overall this looks pretty good. A few minor suggestions:
- I'm not sure that we want to check the names and qualifiers to find the
corresponding `Attribute`s. It is possible that either side could actually
have tables that are aliased differently and thus the `Attribute`s would have
different qualifiers. Instead, I think that it safe to assume that the
analysis phase has checked name and type matching and just find the
corresponding `Attribute` by ordering. I'm thinking something like this:
```scala
/**
* Pushes Project and Filter operations to either side of a Union.
*/
object UnionPushdown extends Rule[LogicalPlan] {
/**
* Rewrites an expression so that it can be pushed to the right side of a
Union operator.
* This method relies on the fact that the output attributes of a union
are always equal to the
* left child's output.
*/
def pushToRight[A <: Expression](e: A, union: Union): A = {
assert(union.left.output.size == union.right.output.size)
// Maps Attributes from the left side to the corresponding Attribute on
the right side.
val rewrites = AttributeMap(union.left.output.zip(union.right.output))
val result = e transform {
case a: Attribute => rewrites(a)
}
// We must promise the compiler that we did not discard the names in
the case of project
// expressions. This is safe since the only transformation is from
Attribute => Attribute.
result.asInstanceOf[A]
}
def apply(plan: LogicalPlan): LogicalPlan = plan transform {
// Push down filter into union
case Filter(condition, u @ Union(left, right)) =>
Union(
Filter(condition, left),
Filter(pushToRight(condition, u), right))
// Push down projection into union
case Project(projectList, u @ Union(left, right)) =>
Union(
Project(projectList, left),
Project(projectList.map(pushToRight(_, u)), right))
}
}
```
- Would also be great to add some tests to `FilterPushdownSuite` and maybe
create a similar `ColumnPruningSuite`.
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