Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/11190#discussion_r53546478
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala ---
@@ -122,7 +125,32 @@ 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[(SparkScalarSubquery,
Future[Array[InternalRow]])]()
+ val allSubqueries = expressions.flatMap(_.collect {case e:
SparkScalarSubquery => 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}: " +
--- End diff --
not 100% sure. maybe it's better to just say more than one, so we don't
need to run the whole plan (e..g i'm thinking maybe we should inject a limit to
subquery)
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