Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/5208#discussion_r28350524
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/basicOperators.scala ---
@@ -41,6 +41,15 @@ case class Project(projectList: Seq[NamedExpression],
child: SparkPlan) extends
val resuableProjection = buildProjection()
iter.map(resuableProjection)
}
+
+ /**
+ * outputOrdering of Project is not always same with child's
outputOrdering if the certain
+ * key is pruned, however, if the key is pruned then we must not require
child using this
+ * ordering from upper layer, only if the ordering would not be changed
by a negative, there
+ * would be a way to keep the ordering.
+ * TODO: we may utilize this feature later to avoid some unnecessary
sorting.
+ */
+ override def outputOrdering: Option[Ordering[Row]] = None
--- End diff --
If you start with a relation `(a,b,c)` and that is sorted by `a`, and you
project `(-a AS newAlias,b,c)`, it is still sorted by `a`.
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