cloud-fan commented on code in PR #42705:
URL: https://github.com/apache/spark/pull/42705#discussion_r1323967912
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/DecorrelateInnerQuery.scala:
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@@ -655,6 +655,39 @@ object DecorrelateInnerQuery extends PredicateHelper {
val newProject = Project(newProjectList ++ referencesToAdd,
newChild)
(newProject, joinCond, outerReferenceMap)
+ case Limit(limit, input) =>
+ // LIMIT K (with potential ORDER BY) is decorrelated by computing
K rows per every
+ // domain value via a row_number() window function. For example,
for a subquery
+ // (SELECT T2.a FROM T2 WHERE T2.b = OuterReference(x) ORDER BY
T2.c LIMIT 3)
+ // -- we need to get top 3 values of T2.a (ordering by T2.c) for
every value of x.
+ // Following our general decorrelation procedure, 'x' is then
replaced by T2.b, so the
+ // subquery is decorrelated as:
+ // SELECT * FROM (
+ // SELECT T2.a, row_number() OVER (PARTITION BY T2.b ORDER BY
T2.c) AS rn FROM T2)
+ // WHERE rn <= 3
+ val (child, ordering) = input match {
+ case Sort(order, _, child) => (child, order)
+ case _ => (input, Seq())
+ }
+ val (newChild, joinCond, outerReferenceMap) =
+ decorrelate(child, parentOuterReferences, aggregated = true,
underSetOp)
+ val collectedChildOuterReferences =
collectOuterReferencesInPlanTree(child)
+ // Add outer references to the PARTITION BY clause
+ val partitionFields =
collectedChildOuterReferences.map(outerReferenceMap(_)).toSeq
+ val orderByFields = replaceOuterReferences(ordering,
outerReferenceMap)
+
+ val rowNumber = WindowExpression(RowNumber(),
+ WindowSpecDefinition(partitionFields, orderByFields,
+ SpecifiedWindowFrame(RowFrame, UnboundedPreceding,
CurrentRow)))
+ val rowNumberAlias = Alias(rowNumber, "rn_" +
NamedExpression.newExprId.id)()
Review Comment:
I'm not sure putting the id in the name is useful. We may refresh attribute
ids in a query plan, and the id in name will be confusing. Shall we just use
`rn`? The EXPLAIN result will print the Alias expr id, so having the id in the
name is not useful.
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