Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/1238#discussion_r14857044
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala ---
@@ -47,6 +47,13 @@ private[sql] abstract class SparkStrategies extends
QueryPlanner[SparkPlan] {
/**
* Uses the ExtractEquiJoinKeys pattern to find joins where at least
some of the predicates can be
* evaluated by matching hash keys.
+ *
+ * This strategy applies a simple optimization based on the estimates of
the physical sizes of
+ * the two join sides. When planning a [[execution.BroadcastHashJoin]],
if one side has an
+ * estimated physical size smaller than the user-settable threshold
+ * `spark.sql.auto.convert.join.size`, the planner would mark it as the
''build'' relation and
--- End diff --
Document it in more explicit ways that a table will get broadcasted
(instead of just saying "build"; you can still "build" in shuffle)
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