[SPARK-55978][SQL][FOLLOWUP] Don't block V2 join pushdown when pushed Sample has fraction=1

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/V2ScanRelationPushDown.scala

@@ -151,11 +151,14 @@ object V2ScanRelationPushDown extends Rule[LogicalPlan] with PredicateHelper {
         rightProjections.forall(_.isInstanceOf[AttributeReference]) &&
         // Cross joins are not supported because they increase the amount of data.
         condition.isDefined &&
-        // Do not push down join if either side has a pushed sample, because
-        // the merged scan builder would silently discard it.
+        // Do not push down join if either side has a pushed sample with
+        // fraction < 1, because the merged scan builder would silently
+        // discard it and change the result. At fraction = 1 the sample is
+        // a no-op on the result set, so dropping it is safe.
         // TODO(SPARK-56504): Extend SupportsPushDownJoin to accept pushed
         //   samples so sources supporting both can handle the composition.
-        leftHolder.pushedSample.isEmpty && rightHolder.pushedSample.isEmpty &&
+        leftHolder.pushedSample.forall(s => s.upperBound - s.lowerBound >= 1.0) &&
+        rightHolder.pushedSample.forall(s => s.upperBound - s.lowerBound >= 1.0) &&
         lBuilder.isOtherSideCompatibleForJoin(rBuilder) =>
       // Process left and right columns in original order
       val (leftSideRequiredColumnsWithAliases, rightSideRequiredColumnsWithAliases) =

sql/core/src/test/scala/org/apache/spark/sql/connector/DataSourceV2TableSampleSuite.scala

@@ -171,9 +171,11 @@ class DataSourceV2TableSampleSuite extends DatasourceV2SQLBase
         val dfNoSample = sql(s"SELECT * FROM $t1 JOIN $t2 ON $t1.id = $t2.id")
         checkJoinPushed(dfNoSample)
 
-        // With SYSTEM sample on one side: join pushdown should be skipped
+        // With a SYSTEM sample (fraction < 1) on one side: join pushdown
+        // should be skipped because the merged scan builder would silently
+        // discard the sample.
         val dfWithSample = sql(
-          s"SELECT * FROM $t1 TABLESAMPLE SYSTEM (100 PERCENT) " +
+          s"SELECT * FROM $t1 TABLESAMPLE SYSTEM (50 PERCENT) " +
           s"JOIN $t2 ON $t1.id = $t2.id")
         checkJoinNotPushed(dfWithSample)
         // The sample should still be pushed down though
@@ -185,6 +187,32 @@ class DataSourceV2TableSampleSuite extends DatasourceV2SQLBase
     }
   }
 
+  test("SPARK-55978: 100% SYSTEM sample does not block join pushdown") {
+    val joinSampleCatalog = "testjoinsample100"
+    registerCatalog(joinSampleCatalog, classOf[InMemoryTableWithJoinAndSampleCatalog])
+    val t1 = s"$joinSampleCatalog.ns.t1"
+    val t2 = s"$joinSampleCatalog.ns.t2"
+    sql(s"CREATE TABLE $t1 (id bigint, data string) USING _")
+    sql(s"CREATE TABLE $t2 (id bigint, data string) USING _")
+    try {
+      sql(s"INSERT INTO $t1 VALUES (1, 'a'), (2, 'b'), (3, 'c')")
+      sql(s"INSERT INTO $t2 VALUES (2, 'x'), (3, 'y'), (4, 'z')")
+      withSQLConf(SQLConf.DATA_SOURCE_V2_JOIN_PUSHDOWN.key -> "true") {
+        // At fraction = 1 the sample is a no-op on the result set, so
+        // dropping it inside the merged scan builder is safe. The guard
+        // in V2ScanRelationPushDown short-circuits and join pushdown
+        // proceeds.
+        val dfWithSample = sql(
+          s"SELECT * FROM $t1 TABLESAMPLE SYSTEM (100 PERCENT) " +
+          s"JOIN $t2 ON $t1.id = $t2.id")
+        checkJoinPushed(dfWithSample)
+      }
+    } finally {
+      sql(s"DROP TABLE IF EXISTS $t1")
+      sql(s"DROP TABLE IF EXISTS $t2")
+    }
+  }
+
   test("SPARK-55978: legacy connector with only 4-arg pushTableSample - BERNOULLI pushes down") {
     val legacyCatalog = "testlegacysample"
     registerCatalog(legacyCatalog, classOf[InMemoryTableWithLegacyTableSampleCatalog])