Packages

object Limit

A constructor for creating a logical limit, which is split into two separate logical nodes: a LocalLimit, which is a partition local limit, followed by a GlobalLimit.

This muds the water for clean logical/physical separation, and is done for better limit pushdown. In distributed query processing, a non-terminal global limit is actually an expensive operation because it requires coordination (in Spark this is done using a shuffle).

In most cases when we want to push down limit, it is often better to only push some partition local limit. Consider the following:

GlobalLimit(Union(A, B))

It is better to do GlobalLimit(Union(LocalLimit(A), LocalLimit(B)))

than Union(GlobalLimit(A), GlobalLimit(B)).

So we introduced LocalLimit and GlobalLimit in the logical plan node for limit pushdown.

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  4. def apply(limitExpr: Expression, child: LogicalPlan): UnaryNode
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  18. def unapply(p: GlobalLimit): Option[(Expression, LogicalPlan)]
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