Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/7988#discussion_r36575758
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/physical/partitioning.scala
---
@@ -75,6 +75,37 @@ case class OrderedDistribution(ordering: Seq[SortOrder])
extends Distribution {
def clustering: Set[Expression] = ordering.map(_.child).toSet
}
+/**
+ * Describes how an operator's output is split across partitions. The
`satisfies`,
+ * `compatibleWith`, and `guarantees` methods describe relationships
between child partitionings,
+ * target partitionings, and [[Distribution]]s. These relations are
described more precisely in
+ * their individual method docs, but at a high level:
+ *
+ * - `satisfies` is a relationship between partitionings and
distributions.
+ * - `compatibleWith` is relationships between an operator's child output
partitionings.
+ * - `guarantees` is a relationship between a child's existing output
partitioning and a target
+ * output partitioning.
+ *
+ * Diagrammatically:
+ *
+ * +--------------+
+ * | Distribution |
+ * +--------------+
+ * ^
+ * |
+ * satisfies
+ * |
+ * +--------------+ +--------------+
+ * | Child | | Target |
+ * +----| Partitioning |----guarantees--->| Partitioning |
+ * | +--------------+ +--------------+
+ * | ^
+ * | |
+ * | compatibleWith
+ * | |
+ * +------------+
+ *
--- End diff --
TODO: document any implications between these relations.
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