fix: report FromUnixTime non-default format as Unsupported

spark/src/main/scala/org/apache/comet/serde/unixtime.scala

@@ -29,21 +29,35 @@ import org.apache.comet.serde.QueryPlanSerde.{exprToProtoInternal, optExprWithFa
 // https://github.com/apache/datafusion/issues/16594
 object CometFromUnixTime extends CometExpressionSerde[FromUnixTime] with CodegenDispatchFallback {
 
-  private val collationReason = DatetimeCollation.reason("from_unixtime")
+  // Applies even to the default format: Comet executes natively but DataFusion's valid timestamp
+  // range differs from Spark, so results can differ outside that range.
+  private val timestampRangeReason =
+    "DataFusion's valid timestamp range differs from Spark" +
+      " (https://github.com/apache/datafusion/issues/16594)"
 
+  // The native (DataFusion) path covers only the default pattern; documented as an unsupported
+  // limitation of that path rather than an incompatibility (see #4575).
   private val formatReason =
-    "Only supports the default datetime format pattern `yyyy-MM-dd HH:mm:ss`." +
-      " DataFusion's valid timestamp range differs from Spark" +
-      " (https://github.com/apache/datafusion/issues/16594)"
+    "Only the default datetime format pattern `yyyy-MM-dd HH:mm:ss` is supported"
 
   override def getIncompatibleReasons(): Seq[String] =
-    Seq(formatReason) ++ DatetimeCollation.incompatibleReasons("from_unixtime")
+    Seq(timestampRangeReason)
 
+  override def getUnsupportedReasons(): Seq[String] =
+    Seq(formatReason)
+
+  // A non-default format has no native (DataFusion) path, so it is `Unsupported`. Because
+  // `CodegenDispatchFallback` is mixed in, an `Unsupported` result still stays in the Comet
+  // pipeline via JVM codegen dispatch (Spark's own `doGenCode`) rather than falling back to Spark.
+  //
+  // Unlike the other datetime expressions, from_unixtime needs no collation guard: a collation can
+  // only appear on the format argument, and any collated format is a non-default format, which is
+  // already `Unsupported` here.
   override def getSupportLevel(expr: FromUnixTime): SupportLevel = {
-    if (DatetimeCollation.hasNonDefaultCollation(expr)) {
-      Incompatible(Some(collationReason))
+    if (expr.format != Literal(TimestampFormatter.defaultPattern)) {
+      Unsupported(Some(formatReason))
     } else {
-      Incompatible(Some(formatReason))
+      Incompatible(Some(timestampRangeReason))
     }
   }
 
@@ -60,7 +74,7 @@ object CometFromUnixTime extends CometExpressionSerde[FromUnixTime] with Codegen
     val timeZone = exprToProtoInternal(Literal(expr.timeZoneId.orNull), inputs, binding)
 
     if (expr.format != Literal(TimestampFormatter.defaultPattern)) {
-      withFallbackReason(expr, "Datetime pattern format is unsupported")
+      withFallbackReason(expr, formatReason)
       None
     } else if (secExpr.isDefined && formatExpr.isDefined) {
       val timestampExpr =

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

@@ -1860,6 +1860,13 @@ class CometExpressionSuite extends CometTestBase with AdaptiveSparkPlanHelper {
             // After fixing these issues, change checkSparkAnswer to checkSparkAnswerAndOperator
             checkSparkAnswer(s"SELECT from_unixtime(_5, 'yyyy') FROM $table $where")
             checkSparkAnswer(s"SELECT from_unixtime(_8, 'yyyy') FROM $table $where")
+            // A non-default format is Unsupported (no native DataFusion path), but at the default
+            // allowIncompatible=false it stays a Comet operator via CodegenDispatchFallback
+            // (Spark's own codegen) rather than falling back to Spark. See #4575.
+            withSQLConf(
+              CometConf.getExprAllowIncompatConfigKey(classOf[FromUnixTime]) -> "false") {
+              checkSparkAnswerAndOperator(s"SELECT from_unixtime(_5, 'yyyy') FROM $table $where")
+            }
             withSQLConf(SESSION_LOCAL_TIMEZONE.key -> "Asia/Kathmandu") {
               checkSparkAnswerAndOperator(s"SELECT from_unixtime(_5) FROM $table $where")
               checkSparkAnswerAndOperator(s"SELECT from_unixtime(_8) FROM $table $where")

spark/src/test/spark-4.0/org/apache/spark/sql/CometCollationSuite.scala

@@ -306,9 +306,12 @@ class CometCollationSuite extends CometTestBase {
   }
 
   test("from_unixtime rejects non-UTF8_BINARY collated format (issue #4646)") {
+    // A collation can only appear on the format argument, so a collated format is a non-default
+    // format. from_unixtime has no native path for non-default formats, so it is reported as
+    // Unsupported (the format reason) rather than Incompatible (the collation reason).
     checkDatetimeFallback(
       "SELECT from_unixtime(_10, _6 COLLATE utf8_lcase) FROM datetime_collation_tbl",
-      "from_unixtime does not support non-UTF8_BINARY collations")
+      "Only the default datetime format pattern `yyyy-MM-dd HH:mm:ss` is supported")
   }
 
   test("make_timestamp rejects non-UTF8_BINARY collated timezone (issue #4646)") {

spark/src/test/spark-4.1/org/apache/spark/sql/CometCollationSuite.scala

@@ -127,9 +127,12 @@ class CometCollationSuite extends CometTestBase {
   }
 
   test("from_unixtime rejects non-UTF8_BINARY collated format (issue #4646)") {
+    // A collation can only appear on the format argument, so a collated format is a non-default
+    // format. from_unixtime has no native path for non-default formats, so it is reported as
+    // Unsupported (the format reason) rather than Incompatible (the collation reason).
     checkDatetimeFallback(
       "SELECT from_unixtime(_10, _6 COLLATE utf8_lcase) FROM datetime_collation_tbl",
-      "from_unixtime does not support non-UTF8_BINARY collations")
+      "Only the default datetime format pattern `yyyy-MM-dd HH:mm:ss` is supported")
   }
 
   test("make_timestamp rejects non-UTF8_BINARY collated timezone (issue #4646)") {