Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/11190#discussion_r53546940
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala ---
@@ -122,7 +125,35 @@ abstract class SparkPlan extends QueryPlan[SparkPlan]
with Logging with Serializ
final def prepare(): Unit = {
if (prepareCalled.compareAndSet(false, true)) {
doPrepare()
+
+ // collect all the subqueries and submit jobs to execute them in
background
+ val queryResults = ArrayBuffer[(ScalarSubquery,
Future[Array[InternalRow]])]()
+ val allSubqueries = expressions.flatMap(_.collect {case e:
ScalarSubquery => e})
+ allSubqueries.foreach { e =>
+ val futureResult = Future {
+ e.plan.executeCollect()
+ }(SparkPlan.subqueryExecutionContext)
+ queryResults += e -> futureResult
+ }
+
children.foreach(_.prepare())
+
+ // fill in the result of subqueries
+ queryResults.foreach {
+ case (e, futureResult) =>
+ val rows = Await.result(futureResult, Duration.Inf)
+ if (rows.length > 1) {
+ sys.error(s"Scalar subquery should return at most one row, but
got ${rows.length}: " +
+ s"${e.query.treeString}")
+ }
+ if (rows.length > 0) {
+ assert(rows(0).numFields == 1, "Analyzer should make sure this
only returns one column")
+ e.updateResult(rows(0).get(0, e.dataType))
+ } else {
+ // the result should be null, since the expression already
have null as default value,
+ // we don't need to update that.
--- End diff --
why not set it explicitly here so the behavior is defined entirely in this
tiny function?
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