Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/11121#discussion_r52821914
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala
---
@@ -91,6 +91,14 @@ abstract class LogicalPlan extends
QueryPlan[LogicalPlan] with Logging {
}
/**
+ * Returns the maximum number of rows that this plan may compute.
+ *
+ * Any operator that a Limit can be pushed passed should override this
function (e.g., Union).
+ * Any operator that can push through a Limit should override this
function (e.g., Project).
+ */
+ def maxRows: Option[Expression] = None
--- End diff --
It's a `def` not a `lazy val`, I think it's fine with constant-folding
rules.
BTW using `Option[Expression]` may be sub-optimal for something like
`Union`, as its `maxRow: Some(children.flatMap(_.maxRows).reduce { (a, b) =>
Add(a, b) })` can't be constant-folded(`maxRow` is a method) and will always be
a non-literal expression, we can't push limit through it.
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