yadavay-amzn commented on code in PR #56101:
URL: https://github.com/apache/spark/pull/56101#discussion_r3769384683


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/joins/BroadcastNearestByJoinExec.scala:
##########
@@ -0,0 +1,149 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution.joins
+
+import java.util.{PriorityQueue => JPriorityQueue}
+
+import org.apache.spark.SparkException
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.plans.{InnerLike, JoinType, LeftOuter, 
NearestByDirection, NearestByDistance}
+import org.apache.spark.sql.catalyst.plans.physical._
+import org.apache.spark.sql.execution.{BinaryExecNode, SparkPlan}
+import org.apache.spark.sql.execution.metric.SQLMetrics
+import org.apache.spark.sql.types.DoubleType
+
+/**
+ * Physical operator for NearestByJoin that avoids materializing the full 
cross product.
+ * For each left row, iterates all broadcast right rows maintaining a bounded 
priority
+ * queue of size k, then emits the top-k matches directly.
+ *
+ * The right side is fully broadcast to all partitions. This operator only 
fires when
+ * the right side fits within [[SQLConf.AUTO_BROADCAST_JOIN_THRESHOLD]]. For 
right tables
+ * exceeding this threshold, the existing cross-product + aggregate rewrite is 
used as
+ * fallback. Tie-breaking among equal ranking values is non-deterministic 
(matches the
+ * existing rewrite behavior).
+ */
+case class BroadcastNearestByJoinExec(
+    left: SparkPlan,
+    right: SparkPlan,
+    joinType: JoinType,
+    numResults: Int,
+    rankingExpression: Expression,
+    direction: NearestByDirection) extends BinaryExecNode {
+
+  override def output: Seq[Attribute] = joinType match {
+    case LeftOuter =>
+      left.output ++ right.output.map(_.withNullability(true))
+    case _: InnerLike =>
+      left.output ++ right.output
+    case other =>
+      throw SparkException.internalError(
+        s"$nodeName does not support join type: $other")
+  }
+
+  override lazy val metrics = Map(
+    "numOutputRows" -> SQLMetrics.createMetric(sparkContext, "number of output 
rows"),
+    "streamedRows" -> SQLMetrics.createMetric(sparkContext, "number of left 
rows processed"))
+
+  override def requiredChildDistribution: Seq[Distribution] =
+    UnspecifiedDistribution :: BroadcastDistribution(IdentityBroadcastMode) :: 
Nil
+
+  override def outputPartitioning: Partitioning = left.outputPartitioning
+
+  override def outputOrdering: Seq[SortOrder] = Nil
+
+  protected override def doExecute(): RDD[InternalRow] = {
+    val broadcastedRight = right.executeBroadcast[Array[InternalRow]]()
+    val numOutput = longMetric("numOutputRows")
+    val k = numResults
+    val isDistance = direction == NearestByDistance
+    val leftOutput = left.output
+    val rightOutput = right.output
+    val rankExpr = rankingExpression
+    val allOutput = output
+
+    left.execute().mapPartitionsInternal { leftIter =>
+      val rightRows = broadcastedRight.value
+      if (rightRows.isEmpty && joinType != LeftOuter) {
+        Iterator.empty
+      } else {
+        val joinedRow = new JoinedRow
+        val rankingProj = UnsafeProjection.create(
+          Seq(Cast(rankExpr, DoubleType)), leftOutput ++ rightOutput)
+        val resultProj = UnsafeProjection.create(allOutput, allOutput)
+        val streamedRowsMetric = longMetric("streamedRows")
+
+        // Hoist heap outside flatMap to reduce GC pressure

Review Comment:
   Done in 338726e2. The operator now pools min(k, rightRows.length) HeapEntry 
objects one time for each partition and uses them again for each left row. A 
new test checks that the count is 2, not near 200.



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CheckAnalysis.scala:
##########
@@ -710,7 +710,12 @@ trait CheckAnalysis extends LookupCatalog with 
QueryErrorsBase with PlanToString
               errorClass = "NEAREST_BY_JOIN.STREAMING_NOT_SUPPORTED",
               messageParameters = Map.empty)
 
-          case j: NearestByJoin if !conf.crossJoinEnabled =>
+          case j: NearestByJoin if !conf.crossJoinEnabled &&
+              !conf.nearestByBroadcastEnabled =>
+            // The cross-join check only applies on the REWRITE path (flag 
OFF) where
+            // a synthetic unconditioned Join node is created. On the OPERATOR 
path
+            // (flag ON), no Join node is built and the operator produces at 
most k rows

Review Comment:
   Done in 338726e2. The comment now notes the flag-on cross-child Python UDF 
fallback.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala:
##########
@@ -424,6 +424,28 @@ abstract class SparkStrategies extends 
QueryPlanner[SparkPlan] {
     }
   }
 
+  /**
+   * Plans NearestByJoin as a BroadcastNearestByJoinExec when the broadcast 
flag is ON.
+   * The optimizer's RewriteNearestByJoin leaves the NearestByJoin node intact 
when the flag

Review Comment:
   Done in 338726e2. The comment now applies only to the nodes that reach the 
strategy.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/joins/BroadcastNearestByJoinExec.scala:
##########
@@ -0,0 +1,225 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution.joins
+
+import java.util.{Comparator, PriorityQueue => JPriorityQueue}
+
+import org.apache.spark.SparkException
+import org.apache.spark.rdd.RDD
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.plans.{InnerLike, JoinType, LeftOuter, 
NearestByDirection, NearestByDistance}
+import org.apache.spark.sql.catalyst.plans.physical._
+import org.apache.spark.sql.catalyst.util.TypeUtils
+import org.apache.spark.sql.execution.{ExplainUtils, SparkPlan}
+import org.apache.spark.sql.execution.metric.SQLMetrics
+
+/**
+ * Heap entry storing an index into the broadcast array alongside its ranking 
value.
+ * Mutable so that evicted entries can be reused: once the heap reaches 
capacity k,
+ * new candidates reuse the polled (worst) entry instead of allocating, 
bounding total
+ * heap-entry allocations to k per left row regardless of right-side size.
+ */
+private[joins] class HeapEntry(var index: Int, var rankingValue: Any)
+
+/**
+ * Physical operator for NearestByJoin that avoids materializing the full 
cross product.
+ * For each left row, iterates all broadcast right rows maintaining a bounded 
priority
+ * queue of size k, then emits the top-k matches directly.
+ *
+ * The right side is fully broadcast unconditionally when
+ * `spark.sql.join.nearestBy.broadcast.enabled` is on.
+ * [[org.apache.spark.sql.catalyst.optimizer.RewriteNearestByJoin]] leaves 
every
+ * [[org.apache.spark.sql.catalyst.plans.logical.NearestByJoin]] intact for 
this operator;

Review Comment:
   Done in 338726e2. The comment now shows the cross-child Python UDF exception.



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/RewriteNearestByJoin.scala:
##########
@@ -85,11 +120,13 @@ object RewriteNearestByJoin extends Rule[LogicalPlan] {
       //    are dropped. When `right` is non-empty every left row already has 
right-row
       //    pairings, so `LEFT OUTER` and `INNER` are equivalent in that case.
       //
-      //    This synthetic join is an unconditioned cross-product, so `NEAREST 
BY` queries
-      //    are subject to `CheckCartesianProducts` and will be rejected when 
the user has
-      //    set `spark.sql.crossJoin.enabled = false`. That is intentional: if 
the user has
-      //    opted out of cross-products, the NEAREST BY rewrite -- which is 
itself a bounded
-      //    cross-product today -- should not silently bypass that choice.
+      //    This synthetic join is an unconditioned cross-product, so on the 
REWRITE path
+      //    (flag OFF) `NEAREST BY` queries are subject to 
`CheckCartesianProducts` and will
+      //    be rejected when the user has set `spark.sql.crossJoin.enabled = 
false`. That is
+      //    intentional: if the user has opted out of cross-products, the 
rewrite -- which is
+      //    itself a bounded cross-product today -- should not silently bypass 
that choice.
+      //    (On the OPERATOR path -- flag ON -- no Join node is built, so

Review Comment:
   Done in 338726e2. The comment now has the correct scope. A cross-child 
Python UDF ranking also enters the rewrite.



##########
sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala:
##########
@@ -2660,6 +2660,19 @@ object SQLConf {
     .booleanConf
     .createWithDefault(true)
 
+  val NEAREST_BY_BROADCAST_ENABLED =
+    buildConf("spark.sql.join.nearestBy.broadcast.enabled")
+      .internal()
+      .doc("When true, NearestByJoin uses a streaming heap operator instead of 
the " +
+        "cross-product + aggregate rewrite. The right side is always 
broadcast, regardless " +
+        "of its size and of spark.sql.autoBroadcastJoinThreshold, so a right 
side too large " +
+        "to broadcast fails the query instead of falling back to the rewrite. 
Because no " +
+        "Join node is built, spark.sql.crossJoin.enabled does not apply on 
this path.")

Review Comment:
   Done in 338726e2. NearestByJoin.hasCrossChildPythonUDF is now the shared 
predicate for the rewrite guard and the CheckAnalysis gate. The gate no longer 
waives the cross-join error for a rewritten node, so 
NEAREST_BY_JOIN.CROSS_JOIN_NOT_ENABLED fires. A new AnalysisErrorSuite test 
checks this.



##########
sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/ValidateSparkPlan.scala:
##########
@@ -58,6 +58,11 @@ object ValidateSparkPlan extends Rule[SparkPlan] {
         validate(buildPlan)
       }
       validate(probePlan)
+    case b: BroadcastNearestByJoinExec =>

Review Comment:
   Done in 338726e2. The operator now returns true in isExplodingJoin and false 
in childrenNeedCompatiblePartitioning. No other AQE matcher needs it.



##########
sql/core/src/test/scala/org/apache/spark/sql/execution/joins/BroadcastNearestByJoinExecSuite.scala:
##########
@@ -0,0 +1,823 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution.joins
+
+import java.sql.Date
+
+import org.apache.spark.sql.{QueryTest, Row}
+import org.apache.spark.sql.execution.adaptive.{AdaptiveSparkPlanExec, 
AdaptiveSparkPlanHelper,
+  BroadcastQueryStageExec}
+import org.apache.spark.sql.functions._
+import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.test.SharedSparkSession
+
+class BroadcastNearestByJoinExecSuite extends QueryTest with SharedSparkSession
+    with AdaptiveSparkPlanHelper {
+
+  import testImplicits._
+
+  private def withStreamingHeap(f: => Unit): Unit = {
+    withSQLConf(
+      SQLConf.NEAREST_BY_BROADCAST_ENABLED.key -> "true") {
+      f
+    }
+  }
+
+  test("empty right table - INNER returns nothing") {
+    withStreamingHeap {
+      val left = spark.range(5).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = spark.range(0).toDF("rid").withColumn("y", lit(0.0))
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 3, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      assert(result.count() == 0)
+    }
+  }
+
+  test("empty right table - LEFT OUTER returns left with nulls") {
+    withStreamingHeap {
+      val left = spark.range(3).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = spark.range(0).toDF("rid").withColumn("y", lit(0.0))
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance", joinType = 
"left_outer")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      assert(result.count() == 3)
+      result.collect().foreach { row =>
+        assert(row.isNullAt(2)) // rid is null
+        assert(row.isNullAt(3)) // y is null
+      }
+    }
+  }
+
+  test("k=1 returns single nearest") {
+    withStreamingHeap {
+      val left = Seq((1, 10.0), (2, 20.0)).toDF("id", "x")
+      val right = Seq((10, 9.0), (11, 15.0), (12, 21.0)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 1, mode = "exact", direction = "distance")
+        .orderBy("id")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      checkAnswer(result, Seq(
+        Row(1, 10.0, 10, 9.0),  // nearest to 10.0 is 9.0
+        Row(2, 20.0, 12, 21.0)  // nearest to 20.0 is 21.0
+      ))
+    }
+  }
+
+  test("k > right table size returns all right rows per left row") {
+    withStreamingHeap {
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      val right = Seq((10, 1.0), (11, 2.0)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 10, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // Only 2 right rows exist, so we get 2 results
+      assert(result.count() == 2)
+      checkAnswer(result.orderBy("rid"), Seq(
+        Row(1, 5.0, 10, 1.0),
+        Row(1, 5.0, 11, 2.0)
+      ))
+    }
+  }
+
+  test("NaN ranking values participate in ordering") {
+    withStreamingHeap {
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      val right = Seq((10, Double.NaN), (11, 3.0), (12, 7.0)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 3, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // NaN participates in natural ordering (sorts after all non-NaN for 
distance)
+      assert(result.count() == 3)
+      checkAnswer(result.orderBy("rid"), Seq(
+        Row(1, 5.0, 10, Double.NaN),
+        Row(1, 5.0, 11, 3.0),
+        Row(1, 5.0, 12, 7.0)
+      ))
+    }
+  }
+
+  test("null ranking values are excluded, not treated as 0.0") {
+    withStreamingHeap {
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      // y=null will produce null ranking expression (abs(5.0 - null) = null)
+      val right = Seq((10, Some(3.0)), (11, None), (12, 
Some(7.0))).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 3, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // null row excluded, only 2 results
+      assert(result.count() == 2)
+      checkAnswer(result.orderBy("rid"), Seq(
+        Row(1, 5.0, 10, 3.0),
+        Row(1, 5.0, 12, 7.0)
+      ))
+    }
+  }
+
+  test("asymmetric - right small left big, operator fires") {
+    withStreamingHeap {
+      val left = spark.range(1000).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = spark.range(50).toDF("rid").withColumn("y", 
col("rid").cast("double") * 20)
+      val df = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 3, mode = "exact", direction = "distance")
+      val plan = df.queryExecution.executedPlan.toString()
+      assert(plan.contains("BroadcastNearestByJoin"),
+        s"Expected BroadcastNearestByJoinExec in plan: $plan")
+      assert(df.count() == 1000 * 3)
+      // Spot check: id=0 (x=0.0) nearest to y values 0,20,40 -> rids 0,1,2
+      val row0 = df.filter(col("id") === 0).orderBy(abs(col("x") - 
col("y"))).collect()
+      assert(row0.length == 3)
+      assert(row0(0).getAs[Long]("rid") == 0L) // y=0, distance=0
+    }
+  }
+
+  test("asymmetric - right exceeds broadcast threshold, broadcast still used 
(flag ON)") {
+    // When the broadcast flag is ON, BroadcastNearestByJoinExec is always 
used regardless
+    // of right-side size. There is no size decision and no fallback; an 
oversized right
+    // side will fail the query at runtime (SPARK-57091).
+    withSQLConf(
+      SQLConf.NEAREST_BY_BROADCAST_ENABLED.key -> "true",
+      SQLConf.AUTO_BROADCASTJOIN_THRESHOLD.key -> "1") {
+      val left = spark.range(10).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = spark.range(50).toDF("rid").withColumn("y", 
col("rid").cast("double"))
+      val df = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance")
+      val plan = df.queryExecution.executedPlan.toString()
+      assert(plan.contains("BroadcastNearestByJoin"),
+        s"Expected BroadcastNearestByJoinExec in plan (flag ON always 
broadcasts): $plan")
+      // Results still correct
+      assert(df.count() == 10 * 2)
+      val row0 = df.filter(col("id") === 0).orderBy(abs(col("x") - 
col("y"))).collect()
+      assert(row0(0).getAs[Long]("rid") == 0L)
+    }
+  }
+
+  test("asymmetric - left small right big, operator fires") {
+    withStreamingHeap {
+      val left = spark.range(10).toDF("id").withColumn("x", 
col("id").cast("double") * 50)
+      val right = spark.range(500).toDF("rid").withColumn("y", 
col("rid").cast("double"))
+      val df = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 5, mode = "exact", direction = "distance")
+      val plan = df.queryExecution.executedPlan.toString()
+      assert(plan.contains("BroadcastNearestByJoin"),
+        s"Expected BroadcastNearestByJoinExec in plan: $plan")
+      assert(df.count() == 10 * 5)
+      // id=0 (x=0.0): nearest are y=0,1,2,3,4
+      val row0 = df.filter(col("id") === 0).orderBy(abs(col("x") - 
col("y"))).collect()
+      assert(row0(0).getAs[Long]("rid") == 0L)
+      assert(row0(4).getAs[Long]("rid") == 4L)
+    }
+  }
+
+  test("asymmetric - both sides moderate, operator fires") {
+    withStreamingHeap {
+      val left = spark.range(200).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = spark.range(200).toDF("rid").withColumn("y", 
col("rid").cast("double") + 0.5)
+      val df = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 3, mode = "exact", direction = "distance")
+      val plan = df.queryExecution.executedPlan.toString()
+      assert(plan.contains("BroadcastNearestByJoin"),
+        s"Expected BroadcastNearestByJoinExec in plan: $plan")
+      assert(df.count() == 200 * 3)
+      // id=100 (x=100.0): nearest y values are 99.5(rid=99), 100.5(rid=100), 
101.5(rid=101)
+      val row100 = df.filter(col("id") === 100).orderBy(abs(col("x") - 
col("y"))).collect()
+      assert(row100.length == 3)
+      assert(row100(0).getAs[Long]("rid") == 99L) // y=99.5, distance=0.5
+    }
+  }
+
+  test("basic correctness - small dataset") {
+    withStreamingHeap {
+      val left = Seq((1, 0.0), (2, 10.0), (3, 20.0)).toDF("id", "x")
+      val right = Seq((100, 1.0), (101, 9.0), (102, 11.0), (103, 19.0), (104, 
25.0))
+        .toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance")
+        .orderBy("id", "y")
+      // id=1 (x=0.0): nearest are y=1.0 (d=1), y=9.0 (d=9)
+      // id=2 (x=10.0): nearest are y=9.0 (d=1), y=11.0 (d=1)
+      // id=3 (x=20.0): nearest are y=19.0 (d=1), y=25.0 (d=5)
+      checkAnswer(result, Seq(
+        Row(1, 0.0, 100, 1.0),
+        Row(1, 0.0, 101, 9.0),
+        Row(2, 10.0, 101, 9.0),
+        Row(2, 10.0, 102, 11.0),
+        Row(3, 20.0, 103, 19.0),
+        Row(3, 20.0, 104, 25.0)
+      ))
+    }
+  }
+
+  test("similarity direction - keeps largest ranking values") {
+    withStreamingHeap {
+      // Higher ranking value = more similar; top-k should be the largest 
values
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      val right = Seq((10, 1.0), (11, 3.0), (12, 8.0), (13, 10.0)).toDF("rid", 
"y")
+      // Use y directly as ranking: higher y = more similar
+      val result = left.nearestByJoin(right, col("y"),
+        numResults = 2, mode = "exact", direction = "similarity")
+        .orderBy(col("y").desc)
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // Top-2 by largest y: rid=13 (y=10.0), rid=12 (y=8.0)
+      checkAnswer(result, Seq(
+        Row(1, 5.0, 13, 10.0),
+        Row(1, 5.0, 12, 8.0)
+      ))
+    }
+  }
+
+  test("integer ranking expression - does not corrupt values") {
+    withStreamingHeap {
+      // x and y are IntegerType, so (x - y) produces IntegerType ranking.
+      // Use negative ranking values to expose getDouble corruption:
+      // int -1 stored as 0x00000000FFFFFFFF reads as a tiny positive double, 
not -1.0.
+      val left = Seq((1, 5)).toDF("id", "x")
+      val right = Seq((10, 6), (11, 3), (12, 100)).toDF("rid", "y")
+      // ranking = x - y: (5-6)=-1, (5-3)=2, (5-100)=-95
+      // direction=similarity means largest ranking wins, so top-2 = 
rid=11(2), rid=10(-1)
+      val result = left.nearestByJoin(right, col("x") - col("y"),
+        numResults = 2, mode = "exact", direction = "similarity")
+        .orderBy((col("x") - col("y")).desc)
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      checkAnswer(result, Seq(
+        Row(1, 5, 11, 3),   // ranking = 2 (largest)
+        Row(1, 5, 10, 6)    // ranking = -1 (second largest)
+      ))
+    }
+  }
+
+  test("tie-breaking - equal distances produce correct count") {
+    withStreamingHeap {
+      // 4 right rows all at distance 1.0 from x=5.0; k=2
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      val right = Seq((10, 4.0), (11, 6.0), (12, 4.0), (13, 6.0)).toDF("rid", 
"y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // All 4 are tied at distance 1.0; we should get exactly k=2 results
+      assert(result.count() == 2)
+      // Each result should have distance 1.0
+      result.collect().foreach { row =>
+        val y = row.getAs[Double]("y")
+        assert(math.abs(5.0 - y) == 1.0)
+      }
+    }
+  }
+
+  // ==========================================================================
+  // SPARK-57091: Tests for ranking comparison fix 
(TypeUtils.getInterpretedOrdering)
+  // ==========================================================================
+
+  test("SPARK-57091: DateType ranking - nearest by earliest date (distance)") {
+    withStreamingHeap {
+      // Date ranking: the old Cast(_, DoubleType) cannot cast dates to double,
+      // producing null rankings and empty results for INNER join.
+      val left = Seq((1, Date.valueOf("2024-06-15"))).toDF("id", "ref_date")
+      val right = Seq(
+        (10, Date.valueOf("2024-06-10")),
+        (11, Date.valueOf("2024-06-20")),
+        (12, Date.valueOf("2024-12-01"))
+      ).toDF("rid", "event_date")
+      // Use event_date as ranking; direction=distance means earliest dates win
+      val result = left.nearestByJoin(right, col("event_date"),
+        numResults = 2, mode = "exact", direction = "distance")
+        .orderBy("event_date")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      checkAnswer(result, Seq(
+        Row(1, Date.valueOf("2024-06-15"), 10, Date.valueOf("2024-06-10")),
+        Row(1, Date.valueOf("2024-06-15"), 11, Date.valueOf("2024-06-20"))
+      ))
+    }
+  }
+
+  test("SPARK-57091: DateType ranking - similarity picks latest date") {
+    withStreamingHeap {
+      // Same DateType scenario but direction=similarity (largest value wins = 
latest date).
+      // The old Cast(_, DoubleType) cannot cast dates, yielding empty results.
+      val left = Seq((1, Date.valueOf("2024-06-15"))).toDF("id", "ref_date")
+      val right = Seq(
+        (10, Date.valueOf("2024-01-01")),
+        (11, Date.valueOf("2024-06-20")),
+        (12, Date.valueOf("2024-12-01"))
+      ).toDF("rid", "event_date")
+      val result = left.nearestByJoin(right, col("event_date"),
+        numResults = 2, mode = "exact", direction = "similarity")
+        .orderBy(col("event_date").desc)
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // Largest (latest) dates: 2024-12-01, 2024-06-20
+      checkAnswer(result, Seq(
+        Row(1, Date.valueOf("2024-06-15"), 12, Date.valueOf("2024-12-01")),
+        Row(1, Date.valueOf("2024-06-15"), 11, Date.valueOf("2024-06-20"))
+      ))
+    }
+  }
+
+  test("SPARK-57091: Long ranking past 2^53 - distinguishes values equal as 
Double") {
+    withStreamingHeap {
+      // Two Long values that differ by 1 but are equal when cast to Double:
+      // Long.MAX_VALUE - 1 and Long.MAX_VALUE both cast to the same Double 
(9.223372036854776E18)
+      // The old Cast(_, DoubleType) approach would see them as equal and pick 
arbitrarily.
+      val v1 = Long.MaxValue - 1 // 9223372036854775806
+      val v2 = Long.MaxValue     // 9223372036854775807
+      // Verify they are indeed equal as Double (precondition)
+      assert(v1.toDouble == v2.toDouble,
+        "precondition: these Longs must be equal as Double to test the fix")
+
+      val left = Seq((1, 0L)).toDF("id", "x")
+      val right = Seq((10, v1), (11, v2)).toDF("rid", "y")
+      // direction=distance: smallest y wins. v1 < v2 as Long but equal as 
Double.
+      val result = left.nearestByJoin(right, col("y"),
+        numResults = 1, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // With proper Long ordering, v1 (smaller) is chosen deterministically
+      checkAnswer(result, Seq(Row(1, 0L, 10, v1)))
+    }
+  }
+
+  test("SPARK-57091: Decimal ranking preserves precision beyond Double") {
+    withStreamingHeap {
+      // Decimals with 18 digits of precision: these are identical when cast 
to Double
+      // but differ in the last digit as Decimal.
+      val d1 = new java.math.BigDecimal("1.000000000000000001")
+      val d2 = new java.math.BigDecimal("1.000000000000000002")
+      // Verify they are equal as Double (precondition)
+      assert(d1.doubleValue() == d2.doubleValue(),
+        "precondition: these Decimals must be equal as Double to test the fix")
+
+      val left = spark.createDataFrame(
+        Seq((1, new java.math.BigDecimal("0")))).toDF("id", "x")
+      val right = spark.createDataFrame(
+        Seq((10, d1), (11, d2))).toDF("rid", "y")
+      // direction=distance: smallest y wins. d1 < d2 as Decimal but equal as 
Double.
+      val result = left.nearestByJoin(right, col("y"),
+        numResults = 1, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      checkAnswer(result, Seq(Row(1, new java.math.BigDecimal("0"), 10, d1)))
+    }
+  }
+
+  test("SPARK-57091: output schema nullability - INNER join has all-nullable 
columns") {
+    withStreamingHeap {
+      // The fix ensures both left and right output attributes are nullable 
for INNER join.
+      // With the old approach, left side would retain original nullability 
(non-nullable for
+      // spark.range), causing schema mismatch with the logical plan.
+      val left = spark.range(3).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = Seq((10, 1.0), (11, 2.0)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 1, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.toString.contains("BroadcastNearestByJoin"),
+        "expected BroadcastNearestByJoinExec in plan")
+      // All output columns must be nullable
+      plan.output.foreach { attr =>
+        assert(attr.nullable,
+          s"column '${attr.name}' should be nullable in INNER join output but 
was not")
+      }
+    }
+  }
+
+  test("SPARK-57091: gate off - broadcast operator does not fire, rewrite path 
used") {
+    // When spark.sql.join.nearestBy.broadcast.enabled is false (default),
+    // the BroadcastNearestByJoinExec must NOT appear and the rewrite path 
must be used.
+    withSQLConf(
+      SQLConf.NEAREST_BY_BROADCAST_ENABLED.key -> "false",
+      SQLConf.CROSS_JOINS_ENABLED.key -> "true") {
+      val left = Seq((1, 10.0), (2, 20.0)).toDF("id", "x")
+      val right = Seq((10, 9.0), (11, 21.0)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 1, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan.toString()
+      assert(!plan.contains("BroadcastNearestByJoin"),
+        "BroadcastNearestByJoinExec must NOT appear when gate is off")
+      // Results are still correct via rewrite path
+      checkAnswer(result.orderBy("id"), Seq(
+        Row(1, 10.0, 10, 9.0),
+        Row(2, 20.0, 11, 21.0)
+      ))
+    }
+  }
+
+  test("SPARK-57091: Long ranking - similarity direction orders correctly past 
2^53") {
+    withStreamingHeap {
+      // v1 = 2^53 - 1, v2 = 2^53, v3 = 2^53 + 1. v2 and v3 are equal as Double
+      // but v1 < v2 < v3 as Long. With k=1 and similarity (largest wins), 
proper
+      // Long ordering deterministically keeps v3 because v3 > v2 strictly.
+      // With a Double cast v2==v3, and the result would be non-deterministic.
+      val v2 = (1L << 53)      // 9007199254740992
+      val v3 = (1L << 53) + 1  // 9007199254740993
+      assert(v2.toDouble == v3.toDouble,
+        "precondition: v2 and v3 must be equal as Double")
+
+      val left = Seq((1, 0L)).toDF("id", "x")
+      // v3 is inserted first; with correct Long comparison the heap retains it
+      // because v3 > v2 strictly, so v2 fails the retention check and is 
never offered.
+      val right = Seq((12, v3), (11, v2)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, col("y"),
+        numResults = 1, mode = "exact", direction = "similarity")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // With proper Long ordering, v3 (larger) must be kept
+      checkAnswer(result, Seq(Row(1, 0L, 12, v3)))
+    }
+  }
+
+  test("SPARK-57091: AQE ValidateSparkPlan accepts 
BroadcastNearestByJoinExec") {
+    // Directly verify that ValidateSparkPlan does not reject a
+    // BroadcastNearestByJoinExec whose right child is a 
BroadcastQueryStageExec.
+    // Without the explicit `case b: BroadcastNearestByJoinExec` in 
ValidateSparkPlan,
+    // the default case recurses into the BroadcastQueryStageExec child and 
throws
+    // InvalidAQEPlanException.
+    import org.apache.spark.sql.execution.adaptive.ValidateSparkPlan
+    withSQLConf(
+      SQLConf.ADAPTIVE_EXECUTION_ENABLED.key -> "true",
+      SQLConf.NEAREST_BY_BROADCAST_ENABLED.key -> "true") {
+      val left = Seq((1, 10.0), (2, 20.0), (3, 30.0)).toDF("id", "x")
+      val right = Seq((10, 9.0), (11, 15.0), (12, 21.0), (13, 
29.0)).toDF("rid", "y")
+      val df = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance")
+      val executedPlan = df.queryExecution.executedPlan
+      // After execution, the plan tree has BroadcastNearestByJoinExec with
+      // BroadcastQueryStageExec as its right child
+      val adaptivePlan = executedPlan match {
+        case aqe: AdaptiveSparkPlanExec =>
+          df.collect() // force execution
+          aqe.executedPlan
+        case other => other
+      }
+      // Find the BroadcastNearestByJoinExec with its BroadcastQueryStageExec 
right child
+      val nearestByOps = collect(adaptivePlan) {
+        case b: BroadcastNearestByJoinExec => b
+      }
+      assert(nearestByOps.nonEmpty,
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
adaptivePlan.treeString)
+      val nb = nearestByOps.head
+      assert(nb.right.isInstanceOf[BroadcastQueryStageExec],
+        s"Expected BroadcastQueryStageExec as right child but got: 
${nb.right.getClass.getName}")
+      // Directly invoke ValidateSparkPlan on the subtree rooted at
+      // BroadcastNearestByJoinExec. This must not throw 
InvalidAQEPlanException.
+      // If the case in ValidateSparkPlan for our operator is missing, this 
throws.
+      ValidateSparkPlan.apply(nb)
+    }
+  }
+
+  test("SPARK-57091: StringType ranking - buffer retention with heap 
eviction") {
+    withStreamingHeap {
+      // Use StringType as the ranking column. UnsafeProjection reuses its 
output buffer,
+      // so UTF8String values point into the mutable buffer. Without .copy(), 
earlier
+      // heap entries get corrupted when the buffer is overwritten on 
subsequent iterations.
+      // Data is ordered so that LATER rows have BETTER (smaller) ranking 
values and
+      // DISPLACE earlier retained entries, exercising the variable-length 
.copy() path.
+      val left = Seq((1, "ref")).toDF("id", "x")
+      val right = Seq(
+        (10, "hhh"), (11, "ggg"), (12, "fff"), (13, "eee"),
+        (14, "ddd"), (15, "ccc"), (16, "bbb"), (17, "aaa")
+      ).toDF("rid", "label")
+      // direction=distance: smallest string wins (lexicographic). k=2 -> 
"aaa", "bbb"
+      // The first entries in the heap are "hhh","ggg" which get displaced by 
later
+      // better values, forcing .copy() of the retained ranking values.
+      val result = left.nearestByJoin(right, col("label"),
+        numResults = 2, mode = "exact", direction = "distance")
+        .orderBy("label")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      checkAnswer(result, Seq(
+        Row(1, "ref", 17, "aaa"),
+        Row(1, "ref", 16, "bbb")
+      ))
+    }
+  }
+
+  test("NaN ranking values under similarity direction") {
+    withStreamingHeap {
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      val right = Seq((10, Double.NaN), (11, 3.0), (12, 8.0), (13, 
10.0)).toDF("rid", "y")
+      // direction=similarity: largest ranking value wins. NaN is largest in 
Java ordering.
+      val result = left.nearestByJoin(right, col("y"),
+        numResults = 2, mode = "exact", direction = "similarity")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // NaN is greatest per Java Double ordering, so top-2 similarity = NaN, 
10.0
+      checkAnswer(result.orderBy(col("y").desc_nulls_last), Seq(
+        Row(1, 5.0, 10, Double.NaN),
+        Row(1, 5.0, 13, 10.0)
+      ))
+    }
+  }
+
+  test("null in non-ranking right column propagates correctly") {
+    withStreamingHeap {
+      val left = Seq((1, 5.0)).toDF("id", "x")
+      // "label" column has nulls but is NOT the ranking column
+      val right = Seq(
+        (10, 3.0, Some("a")),
+        (11, 7.0, None),
+        (12, 100.0, Some("c"))
+      ).toDF("rid", "y", "label")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance")
+        .orderBy("rid")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // Top-2 nearest: rid=10 (d=2.0, label="a"), rid=11 (d=2.0, label=null)
+      checkAnswer(result, Seq(
+        Row(1, 5.0, 10, 3.0, "a"),
+        Row(1, 5.0, 11, 7.0, null)
+      ))
+    }
+  }
+
+  test("SPARK-57091: non-deterministic ranking expression (rand()) does not 
throw") {
+    withStreamingHeap {
+      // A non-deterministic ranking expression must have its projection 
initialized
+      // with the partition index before evaluation. Without initialization, 
rand() throws
+      // "Nondeterministic expression ... has not been initialized".
+      val left = spark.range(10).toDF("id").withColumn("x", 
col("id").cast("double"))
+      val right = Seq((10, 1.0), (11, 2.0), (12, 3.0)).toDF("rid", "y")
+      // Use rand() as ranking: non-deterministic, returns Double
+      val result = left.nearestByJoin(right, rand(),
+        numResults = 2, mode = "exact", direction = "similarity")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      // Each of 10 left rows should get 2 right rows (k=2, right has 3 rows)
+      assert(result.count() == 20)
+    }
+  }
+
+  test("SPARK-57091: DSv2 right side does not throw INTERNAL_ERROR with flag 
ON") {
+    // Regression: before the stats-free fix, reading right.stats.sizeInBytes 
in the
+    // optimizer's FinishAnalysis batch triggered computeStats on a DSv2 
source before
+    // filter/partition pushdown completed, throwing [INTERNAL_ERROR]. With 
the fix,
+    // the optimizer no longer reads stats -- the NearestByJoin node is left 
intact for
+    // the planner, which unconditionally plans BroadcastNearestByJoinExec.
+    withSQLConf(
+      "spark.sql.catalog.testcat" ->
+        
classOf[org.apache.spark.sql.connector.catalog.InMemoryTableCatalog].getName,
+      SQLConf.NEAREST_BY_BROADCAST_ENABLED.key -> "true") {
+      spark.sql(
+        """CREATE TABLE testcat.right_tbl (rid INT, y DOUBLE)
+          |USING foo""".stripMargin)
+      try {
+        spark.sql("INSERT INTO testcat.right_tbl VALUES (10, 1.0), (11, 5.0), 
(12, 9.0)")
+        val left = Seq((1, 3.0), (2, 7.0)).toDF("id", "x")
+        val right = spark.table("testcat.right_tbl")
+        val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+          numResults = 2, mode = "exact", direction = "distance")
+        val plan = result.queryExecution.executedPlan
+        assert(plan.treeString.contains("BroadcastNearestByJoin"),
+          "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+        // Verify correct results:
+        // id=1(x=3.0) nearest y=1.0(d=2),5.0(d=2)
+        // id=2(x=7.0) nearest y=5.0(d=2),9.0(d=2)
+        assert(result.count() == 4)
+        val row1 = result.filter(col("id") === 1).orderBy(abs(col("x") - 
col("y"))).collect()
+        assert(row1.length == 2)
+      } finally {
+        spark.sql("DROP TABLE IF EXISTS testcat.right_tbl")
+      }
+    }
+  }
+
+  test("SPARK-57091: partitioned right table with flag ON uses 
BroadcastNearestByJoin") {
+    // Regression: before the stats-free fix, partitioned file tables reported 
inflated
+    // sizeInBytes before filter pushdown. With the unconditional broadcast 
contract,
+    // the planner always plans BroadcastNearestByJoinExec when the flag is ON.
+    withStreamingHeap {
+      withTempPath { dir =>
+        // Create partitioned parquet data
+        val data = (1 to 100).map(i => (i % 5, i, i.toDouble))
+        spark.createDataFrame(data).toDF("part", "rid", "y")
+          .write.partitionBy("part").parquet(dir.getAbsolutePath)
+
+        val right = spark.read.parquet(dir.getAbsolutePath).filter(col("part") 
=== 0)
+        val left = Seq((1, 10.0), (2, 50.0)).toDF("id", "x")
+        val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+          numResults = 3, mode = "exact", direction = "distance")
+        val plan = result.queryExecution.executedPlan
+        assert(plan.treeString.contains("BroadcastNearestByJoin"),
+          "Expected BroadcastNearestByJoinExec for partitioned right but 
got:\n" +
+          plan.treeString)
+        // part=0 has values 5,10,15,...,100 (20 rows). Each left row gets 3 
nearest.
+        assert(result.count() == 6)
+      }
+    }
+  }
+
+  // ==========================================================================
+  // F7: crossJoin.enabled divergence -- operator path does not require it
+  // ==========================================================================
+
+  test("SPARK-57091: flag ON succeeds without crossJoin.enabled") {
+    // When the broadcast flag is ON, no Join node is built so 
CheckCartesianProducts
+    // does not apply. NEAREST BY should succeed even with crossJoin.enabled = 
false.
+    withSQLConf(
+      SQLConf.NEAREST_BY_BROADCAST_ENABLED.key -> "true",
+      SQLConf.CROSS_JOINS_ENABLED.key -> "false") {
+      val left = Seq((1, 10.0), (2, 20.0)).toDF("id", "x")
+      val right = Seq((10, 9.0), (11, 15.0), (12, 21.0)).toDF("rid", "y")
+      val result = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 2, mode = "exact", direction = "distance")
+      val plan = result.queryExecution.executedPlan
+      assert(plan.treeString.contains("BroadcastNearestByJoin"),
+        "Expected BroadcastNearestByJoinExec in plan but got:\n" + 
plan.treeString)
+      checkAnswer(result.filter(col("id") === 1).orderBy(abs(col("x") - 
col("y"))), Seq(
+        Row(1, 10.0, 10, 9.0),
+        Row(1, 10.0, 11, 15.0)
+      ))
+      checkAnswer(result.filter(col("id") === 2).orderBy(abs(col("x") - 
col("y"))), Seq(
+        Row(2, 20.0, 12, 21.0),
+        Row(2, 20.0, 11, 15.0)
+      ))
+    }
+  }
+
+  // ==========================================================================
+  // EXPLAIN FORMATTED output
+  // ==========================================================================
+
+  test("SPARK-57091: EXPLAIN FORMATTED shows ranking, k, and direction") {
+    withStreamingHeap {
+      val left = Seq((1, 10.0)).toDF("id", "x")
+      val right = Seq((10, 9.0)).toDF("rid", "y")
+      val df = left.nearestByJoin(right, abs(col("x") - col("y")),
+        numResults = 3, mode = "exact", direction = "distance")
+      val explain = df.queryExecution.explainString(
+        org.apache.spark.sql.execution.FormattedMode)
+      assert(explain.contains("NumResults: 3"),
+        s"EXPLAIN FORMATTED should contain 'NumResults: 3'\n$explain")
+      assert(explain.contains("Direction: NearestByDistance"),
+        s"EXPLAIN FORMATTED should contain 'Direction: 
NearestByDistance'\n$explain")
+      assert(explain.contains("Ranking:"),
+        s"EXPLAIN FORMATTED should contain 'Ranking:'\n$explain")
+      assert(explain.contains("Join type: Inner"),
+        s"EXPLAIN FORMATTED should contain 'Join type: Inner'\n$explain")
+    }
+  }
+
+  // ==========================================================================
+  // Cross-child Python UDF fallback to rewrite path
+  // ==========================================================================
+
+  test("SPARK-57091: cross-child Python UDF in ranking routes through rewrite 
path") {

Review Comment:
   Done in 338726e2. test_nearest_by_join.py now runs a two-sided and a 
right-side-only scalar Python UDF ranking, and checks parity across the flag.



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