[GitHub] [spark] allisonwang-db commented on a diff in pull request #42705: [SPARK-36191][SQL] Handle limit and order by in correlated scalar (lateral) subqueries

["allisonwang-db (via GitHub)" <gi...@apache.org>]

allisonwang-db commented on code in PR #42705:
URL: https://github.com/apache/spark/pull/42705#discussion_r1323305701


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CheckAnalysis.scala:
##########
@@ -1405,6 +1405,11 @@ trait CheckAnalysis extends PredicateHelper with LookupCatalog with QueryErrorsB
           failOnInvalidOuterReference(g)
           checkPlan(g.child, aggregated, canContainOuter)
 
+        // Correlated subquery can have a LIMIT clause
+        case l@Limit(_, input) =>

Review Comment:
   ```suggestion
           case l @ Limit(_, input) =>
   ```



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/DecorrelateInnerQuery.scala:
##########
@@ -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 (

Review Comment:
   Great explanation here!



##########
sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/DecorrelateInnerQuerySuite.scala:
##########
@@ -59,6 +59,25 @@ class DecorrelateInnerQuerySuite extends PlanTest {
     joinCond.zip(conditions).foreach(e => compareExpressions(e._1, e._2))
   }
 
+  private def check(
+                     outputPlan: LogicalPlan,
+                     joinCond: Seq[Expression],
+                     correctAnswer: LogicalPlan,
+                     conditions: Seq[Expression]): Unit = {
+    assert(!hasOuterReferences(outputPlan))

Review Comment:
   nit: indent
   ```suggestion
     private def check(
         outputPlan: LogicalPlan,
         joinCond: Seq[Expression],
         correctAnswer: LogicalPlan,
         conditions: Seq[Expression]): Unit = {
       assert(!hasOuterReferences(outputPlan))
   ```



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