Github user yhuai commented on a diff in the pull request:
https://github.com/apache/spark/pull/9788#discussion_r45166467
--- Diff: core/src/main/scala/org/apache/spark/util/SizeEstimator.scala ---
@@ -32,6 +32,16 @@ import org.apache.spark.annotation.DeveloperApi
import org.apache.spark.util.collection.OpenHashSet
/**
+ * A trait that allows a class to give [[SizeEstimator]] more accurate
size estimation.
+ * When a class extends it, [[SizeEstimator]] will query the
`estimatedSize` first.
+ * If `estimatedSize` does not return [[None]], [[SizeEstimator]] will use
the returned size
+ * as the size of the object. Otherwise, [[SizeEstimator]] will do the
estimation work.
+ */
+private[spark] trait SizeEstimation {
+ def estimatedSize: Option[Long]
--- End diff --
I'd like to get it merged first if there is no fundamental issue. So, we
can unblock the preview package. I can make the change if we prefer to change
`BytesToBytesMap` instead of `UnsafeHashedRelation`. I agree returning a
`Option` is weird. But, I feel if it is possible, we should prefer changing
`UnsafeHashedRelation` because it is the one used as the broadcast variable.
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