Github user frreiss commented on a diff in the pull request:
https://github.com/apache/spark/pull/13155#discussion_r63798715
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -1648,16 +1648,56 @@ object RewriteCorrelatedScalarSubquery extends
Rule[LogicalPlan] {
}
/**
+ * Statically evaluate an expression containing one or more aggregates
on an empty input.
+ */
+ private def evalOnZeroTups(expr : Expression) : Option[Any] = {
+ // AggregateExpressions are Unevaluable, so we need to replace all
aggregates
+ // in the expression with the value they would return for zero input
tuples.
+ val rewrittenExpr = expr transform {
+ case a @ AggregateExpression(aggFunc, _, _, resultId) =>
+ val resultLit = aggFunc.defaultResult match {
+ case Some(lit) => lit
+ case None => Literal.default(NullType)
+ }
+ Alias(resultLit, "aggVal") (exprId = resultId)
+ }
+ Option(rewrittenExpr.eval())
+ }
+
+ /**
* Construct a new child plan by left joining the given subqueries to a
base plan.
*/
private def constructLeftJoins(
child: LogicalPlan,
subqueries: ArrayBuffer[ScalarSubquery]): LogicalPlan = {
subqueries.foldLeft(child) {
case (currentChild, ScalarSubquery(query, conditions, _)) =>
+ val aggOutputExpr =
query.asInstanceOf[Aggregate].aggregateExpressions.head
--- End diff --
Update: The relevant upstream checks are in CheckAnalysis.checkAnalysis():
```scala
case s @ ScalarSubquery(query, conditions, _) if conditions.nonEmpty =>
...
def cleanQuery(p: LogicalPlan): LogicalPlan = p match {
case SubqueryAlias(_, child) => cleanQuery(child)
case Project(_, child) => cleanQuery(child)
case child => child
}
cleanQuery(query) match {
case a: Aggregate => checkAggregate(a)
case Filter(_, a: Aggregate) => checkAggregate(a)
case fail => failAnalysis(s"Correlated scalar subqueries must be
Aggregated: $fail")
}
```
These checks will accept a chain of operators above the Aggregate that match
the regex:
(SubqueryAlias|Project)*(Filter)?
It doesn't look like AstBuilder will generate a SubqueryAlias nested
immediately inside a SubqueryAlias, but all other matches of the above regex
are possible.
I'll update `evalOnZeroTups()` above to rewrite combinations of Project,
Filter, and Aggregate into an expression, then statically evaluate the
expression.
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