Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/10451#discussion_r48581020
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -80,6 +81,33 @@ abstract class Optimizer extends
RuleExecutor[LogicalPlan] {
object DefaultOptimizer extends Optimizer
/**
+ * Pushes down Limit for reducing the amount of returned data.
+ *
+ * 1. Adding Extra Limit beneath the operations, including Union All.
+ * 2. Project is pushed through Limit in the rule ColumnPruning
+ *
+ * Any operator that a Limit can be pushed passed should override the
maxRows function.
+ */
+object PushDownLimit extends Rule[LogicalPlan] {
+
+ def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+
+ // Adding extra Limit below UNION ALL iff both left and right childs
are not Limit or
+ // do not have Limit descendants. This heuristic is valid assuming
there does not exist
+ // any Limit push-down rule that is unable to infer the value of
maxRows.
+ // Note, right now, Union means UNION ALL, which does not de-duplicate
rows. So, it is
+ // safe to pushdown Limit through it. Once we add UNION DISTINCT, we
will not be able to
+ // pushdown Limit.
+ case Limit(exp, Union(left, right))
+ if left.maxRows.isEmpty || right.maxRows.isEmpty =>
--- End diff --
Yeah, you are right. : )
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