jchen5 commented on code in PR #46839:
URL: https://github.com/apache/spark/pull/46839#discussion_r1623678864
##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala:
##########
@@ -249,6 +250,73 @@ object SubExprUtils extends PredicateHelper {
}
}
}
+
+ /**
+ * Returns the inner query attributes that are guaranteed to have a single
value for each
+ * outer row. Therefore, a scalar subquery is allowed to group-by on these
attributes.
+ * We can derive these from correlated equality predicates, though we need
to take care about
+ * propagating this through operators like OUTER JOIN or UNION.
+ *
+ * Positive examples: x = outer(a) AND y = outer(b)
+ * Negative examples:
+ * - x <= outer(a)
+ * - x + y = outer(a)
+ * - x = outer(a) OR y = outer(b)
+ * - y = outer(b) + 1 (this and similar expressions could be supported, but
very carefully)
+ * - An equality under the right side of a LEFT OUTER JOIN, e.g.
+ * select *, (select count(*) from y left join
+ * (select * from z where z1 = x1) sub on y2 = z2 group by z1) from x;
+ * - An equality under UNION e.g.
+ * select *, (select count(*) from
+ * (select * from y where y1 = x1 union all select * from y) group by
y1) from x;
+ */
+ def getCorrelatedEquivalentInnerColumns(plan: LogicalPlan): AttributeSet = {
+ plan match {
+ case Filter(cond, child) =>
+ val correlated = AttributeSet(splitConjunctivePredicates(cond)
+ .filter(containsOuter) // TODO: can remove this line to allow e.g.
where x = 1 group by x
+ .filter(DecorrelateInnerQuery.canPullUpOverAgg)
+ .flatMap(_.references))
+ correlated ++ getCorrelatedEquivalentInnerColumns(child)
+
+ case Join(left, right, joinType, _, _) =>
+ joinType match {
+ case _: InnerLike =>
+ AttributeSet(plan.children.flatMap(child =>
getCorrelatedEquivalentInnerColumns(child)))
+ case LeftOuter => getCorrelatedEquivalentInnerColumns(left)
+ case RightOuter => getCorrelatedEquivalentInnerColumns(right)
+ case FullOuter => AttributeSet.empty
+ case LeftSemi => getCorrelatedEquivalentInnerColumns(left)
+ case LeftAnti => getCorrelatedEquivalentInnerColumns(left)
+ case _ => AttributeSet.empty
+ }
+
+ case _: Union => AttributeSet.empty
+ case Except(left, right, _) => getCorrelatedEquivalentInnerColumns(left)
+
+ case
+ _: Aggregate |
+ _: Distinct |
+ _: Intersect |
+ _: GlobalLimit |
+ _: LocalLimit |
+ _: Offset |
+ _: Project |
+ _: Repartition |
+ _: RepartitionByExpression |
+ _: RebalancePartitions |
+ _: Sample |
+ _: Sort |
+ _: Window |
+ _: Tail |
+ _: WithCTE |
+ _: Range |
+ _: SubqueryAlias =>
+ AttributeSet(plan.children.flatMap(child =>
getCorrelatedEquivalentInnerColumns(child)))
+
+ case _ => AttributeSet.empty
Review Comment:
The list of operators handled here is by no means comprehensive and ensuring
it covers enough is tricky. I used the list in LogicalPlanVisitor as a starting
point, but in my testing I discovered that e.g. SubqueryAlias also needs to be
handled to cover cases with FROM subqueries inside the scalar subquery.
Suggestions on other important operators to handle or other potential
approaches welcome.
(In the long run I think we need to replace this entire check with a runtime
check as described in https://issues.apache.org/jira/browse/SPARK-48501, but
that's highly nontrivial)
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