Github user JoshRosen commented on a diff in the pull request: https://github.com/apache/spark/pull/11121#discussion_r52246581 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala --- @@ -130,6 +131,48 @@ object EliminateSerialization extends Rule[LogicalPlan] { } /** + * Pushes down [[LocalLimit]] beneath UNION ALL and beneath the streamed inputs of outer joins. + */ +object LimitPushDown extends Rule[LogicalPlan] { + + private def stripGlobalLimitIfPresent(plan: LogicalPlan): LogicalPlan = { + plan match { + case GlobalLimit(expr, child) => child + case _ => plan + } + } + + private def maybePushLimit(limitExp: Expression, plan: LogicalPlan): LogicalPlan = { + (limitExp, plan.maxRows) match { + case (IntegerLiteral(maxRow), Some(IntegerLiteral(childMaxRows))) if maxRow < childMaxRows => + LocalLimit(limitExp, stripGlobalLimitIfPresent(plan)) + case (_, None) => + LocalLimit(limitExp, stripGlobalLimitIfPresent(plan)) + case _ => plan + } + } + + def apply(plan: LogicalPlan): LogicalPlan = plan transform { + // Adding extra Limits below UNION ALL for children which are not Limit or do not have Limit + // descendants whose maxRow is larger. 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, however, we will not be able to + // pushdown Limit. + case LocalLimit(exp, Union(children)) => + LocalLimit(exp, Union(children.map(maybePushLimit(exp, _)))) + case LocalLimit(exp, join @ Join(left, right, joinType, condition)) => + joinType match { + case RightOuter => join.copy(right = maybePushLimit(exp, right)) + case LeftOuter => join.copy(left = maybePushLimit(exp, left)) + case FullOuter => + join.copy(left = maybePushLimit(exp, left), right = maybePushLimit(exp, right)) --- End diff -- Also, I wonder whether we should check whether `maxRows` is defined rather than checking whether the outer join's children are `Limit`s, since that frees us from having to reason about whether the limit could be further pushed. On the other hand, if we always leave the original `LocalLimit` in place then I don't think we currently need to worry about the limit being further pushed down to a point where the child would no longer be a limit.
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