feat: allow native Iceberg scans with non-identity transform residuals

spark/src/main/scala/org/apache/comet/rules/CometScanRule.scala

@@ -478,29 +478,35 @@ case class CometScanRule(session: SparkSession) extends Rule[SparkPlan] with Com
             false
           }
 
-        // Check for unsupported transform functions in residual expressions
-        // iceberg-rust can only handle identity transforms in residuals; all other transforms
-        // (truncate, bucket, year, month, day, hour) must fall back to Spark
+        // Check for transform functions in residual expressions
+        // Non-identity transforms (truncate, bucket, year, month, day, hour) in residuals
+        // are now supported - they skip row-group filtering and are handled
+        // post-scan by CometFilter.
+        // This is less optimal than row-group filtering but still allows native execution.
         val transformFunctionsSupported =
           try {
             IcebergReflection.findNonIdentityTransformInResiduals(metadata.tasks) match {
               case Some(transformType) =>
-                // Found unsupported transform
-                fallbackReasons +=
-                  s"Iceberg transform function '$transformType' in residual expression " +
-                    "is not yet supported by iceberg-rust. " +
-                    "Only identity transforms are supported."
-                false
+                // Found non-identity transform - log info and continue with native scan
+                // Row-group filtering will skip these predicates, but post-scan
+                // filtering will apply
+                logInfo(
+                  s"Iceberg residual contains transform '$transformType' - " +
+                    "row-group filtering will skip this predicate, " +
+                    "post-scan filtering by CometFilter will apply instead.")
+                true // Allow native execution
               case None =>
-                // No unsupported transforms found - safe to use native execution
+                // No non-identity transforms - optimal row-group filtering will apply
                 true
             }
           } catch {
             case e: Exception =>
-              // Reflection failure - cannot verify safety, must fall back
-              fallbackReasons += "Iceberg reflection failure: Could not check for " +
-                s"transform functions in residuals: ${e.getMessage}"
-              false
+              // Reflection failure - log warning but allow native execution
+              // The predicate conversion will handle unsupported cases gracefully
+              logWarning(
+                s"Could not check for transform functions in residuals: ${e.getMessage}. " +
+                  "Continuing with native scan.")
+              true
           }
 
         // Check for unsupported struct types in delete files

spark/src/test/scala/org/apache/comet/CometIcebergNativeSuite.scala

@@ -2283,6 +2283,160 @@ class CometIcebergNativeSuite extends CometTestBase with RESTCatalogHelper {
     }
   }
 
+  // Tests for non-identity transform residuals feature
+  // These tests verify that native Iceberg scans work when residual expressions
+  // contain non-identity transforms (truncate, bucket, year, month, day, hour).
+  // Previously these would fall back to Spark, but now they're supported with
+  // post-scan filtering via CometFilter.
+
+  test("non-identity transform residual - truncate transform allows native scan") {
+    assume(icebergAvailable, "Iceberg not available in classpath")
+
+    withTempIcebergDir { warehouseDir =>
+      withSQLConf(
+        "spark.sql.catalog.test_cat" -> "org.apache.iceberg.spark.SparkCatalog",
+        "spark.sql.catalog.test_cat.type" -> "hadoop",
+        "spark.sql.catalog.test_cat.warehouse" -> warehouseDir.getAbsolutePath,
+        CometConf.COMET_ENABLED.key -> "true",
+        CometConf.COMET_EXEC_ENABLED.key -> "true",
+        CometConf.COMET_ICEBERG_NATIVE_ENABLED.key -> "true") {
+
+        // Create table partitioned by truncate transform
+        // When filtering by exact value (e.g., name = 'alpha_1'), Iceberg creates
+        // a residual expression because truncate(5, name) can't fully evaluate this
+        spark.sql("""
+          CREATE TABLE test_cat.db.truncate_residual_test (
+            id INT,
+            name STRING
+          ) USING iceberg
+          PARTITIONED BY (truncate(5, name))
+        """)
+
+        spark.sql("""
+          INSERT INTO test_cat.db.truncate_residual_test VALUES
+          (1, 'alpha_1'), (2, 'alpha_2'), (3, 'alpha_3'),
+          (4, 'bravo_1'), (5, 'bravo_2'), (6, 'charlie_1')
+        """)
+
+        // This filter creates a residual with truncate transform
+        // The partition can narrow down to 'alpha' prefix, but exact match
+        // requires post-scan filtering
+        checkIcebergNativeScan(
+          "SELECT * FROM test_cat.db.truncate_residual_test WHERE name = 'alpha_2' ORDER BY id")
+
+        // Verify correct results
+        val result = spark
+          .sql("SELECT * FROM test_cat.db.truncate_residual_test WHERE name = 'alpha_2'")
+          .collect()
+        assert(result.length == 1, s"Expected 1 row, got ${result.length}")
+        assert(result(0).getInt(0) == 2, s"Expected id=2, got ${result(0).getInt(0)}")
+
+        spark.sql("DROP TABLE test_cat.db.truncate_residual_test")
+      }
+    }
+  }
+
+  test("non-identity transform residual - bucket transform allows native scan") {
+    assume(icebergAvailable, "Iceberg not available in classpath")
+
+    withTempIcebergDir { warehouseDir =>
+      withSQLConf(
+        "spark.sql.catalog.test_cat" -> "org.apache.iceberg.spark.SparkCatalog",
+        "spark.sql.catalog.test_cat.type" -> "hadoop",
+        "spark.sql.catalog.test_cat.warehouse" -> warehouseDir.getAbsolutePath,
+        CometConf.COMET_ENABLED.key -> "true",
+        CometConf.COMET_EXEC_ENABLED.key -> "true",
+        CometConf.COMET_ICEBERG_NATIVE_ENABLED.key -> "true") {
+
+        // Create table partitioned by bucket transform
+        // When filtering by exact id value, Iceberg creates a residual expression
+        // because bucket(4, id) maps multiple ids to the same bucket
+        spark.sql("""
+          CREATE TABLE test_cat.db.bucket_residual_test (
+            id INT,
+            value DOUBLE
+          ) USING iceberg
+          PARTITIONED BY (bucket(4, id))
+        """)
+
+        spark.sql("""
+          INSERT INTO test_cat.db.bucket_residual_test
+          SELECT id, CAST(id * 1.5 AS DOUBLE) as value
+          FROM range(100)
+        """)
+
+        // This filter creates a residual with bucket transform
+        // The partition pruning uses bucket hash, but exact id match
+        // requires post-scan filtering
+        checkIcebergNativeScan(
+          "SELECT * FROM test_cat.db.bucket_residual_test WHERE id = 42 ORDER BY id")
+
+        // Verify correct results
+        val result =
+          spark.sql("SELECT * FROM test_cat.db.bucket_residual_test WHERE id = 42").collect()
+        assert(result.length == 1, s"Expected 1 row, got ${result.length}")
+        assert(result(0).getInt(0) == 42, s"Expected id=42, got ${result(0).getInt(0)}")
+
+        spark.sql("DROP TABLE test_cat.db.bucket_residual_test")
+      }
+    }
+  }
+
+  test("non-identity transform residual - year transform allows native scan") {
+    assume(icebergAvailable, "Iceberg not available in classpath")
+
+    withTempIcebergDir { warehouseDir =>
+      withSQLConf(
+        "spark.sql.catalog.test_cat" -> "org.apache.iceberg.spark.SparkCatalog",
+        "spark.sql.catalog.test_cat.type" -> "hadoop",
+        "spark.sql.catalog.test_cat.warehouse" -> warehouseDir.getAbsolutePath,
+        CometConf.COMET_ENABLED.key -> "true",
+        CometConf.COMET_EXEC_ENABLED.key -> "true",
+        CometConf.COMET_ICEBERG_NATIVE_ENABLED.key -> "true") {
+
+        // Create table partitioned by year transform
+        // When filtering by exact date, Iceberg creates a residual expression
+        // because year(event_date) groups all dates in a year together
+        spark.sql("""
+          CREATE TABLE test_cat.db.year_residual_test (
+            id INT,
+            event_date DATE,
+            data STRING
+          ) USING iceberg
+          PARTITIONED BY (year(event_date))
+        """)
+
+        spark.sql("""
+          INSERT INTO test_cat.db.year_residual_test VALUES
+          (1, DATE '2023-01-15', 'jan'),
+          (2, DATE '2023-06-20', 'jun'),
+          (3, DATE '2023-12-25', 'dec'),
+          (4, DATE '2024-01-10', 'new_year'),
+          (5, DATE '2024-07-04', 'july')
+        """)
+
+        // This filter creates a residual with year transform
+        // Partition pruning narrows to 2023, but exact date match
+        // requires post-scan filtering
+        checkIcebergNativeScan(
+          "SELECT * FROM test_cat.db.year_residual_test WHERE event_date = DATE '2023-06-20'")
+
+        // Verify correct results
+        val result = spark
+          .sql(
+            "SELECT * FROM test_cat.db.year_residual_test WHERE event_date = DATE '2023-06-20'")
+          .collect()
+        assert(result.length == 1, s"Expected 1 row, got ${result.length}")
+        assert(result(0).getInt(0) == 2, s"Expected id=2, got ${result(0).getInt(0)}")
+        assert(
+          result(0).getString(2) == "jun",
+          s"Expected data='jun', got ${result(0).getString(2)}")
+
+        spark.sql("DROP TABLE test_cat.db.year_residual_test")
+      }
+    }
+  }
+
   // Helper to create temp directory
   def withTempIcebergDir(f: File => Unit): Unit = {
     val dir = Files.createTempDirectory("comet-iceberg-test").toFile