chore: fix spark ansi sum incompatibility message

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

@@ -209,8 +209,6 @@ object CometAverage extends CometAggregateExpressionSerde[Average] {
 
 object CometSum extends CometAggregateExpressionSerde[Sum] {
 
-  override def getIncompatibleReasons(): Seq[String] = Seq("Falls back to Spark in ANSI mode.")
-
   override def convert(
       aggExpr: AggregateExpression,
       sum: Sum,

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

@@ -33,10 +33,6 @@ import org.apache.comet.serde.QueryPlanSerde.{createBinaryExpr, exprToProtoInter
 
 object CometStringRepeat extends CometExpressionSerde[StringRepeat] {
 
-  override def getCompatibleNotes(): Seq[String] = Seq(
-    "A negative argument for the number of times to repeat throws an exception" +
-      " instead of returning an empty string as Spark does")
-
   override def convert(
       expr: StringRepeat,
       inputs: Seq[Attribute],

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

@@ -21,6 +21,7 @@ package org.apache.comet.serde
 
 import org.apache.spark.sql.catalyst.expressions.{Attribute, FromUnixTime, Literal}
 import org.apache.spark.sql.catalyst.util.TimestampFormatter
+import org.apache.spark.sql.types.StringType
 
 import org.apache.comet.CometSparkSessionExtensions.withInfo
 import org.apache.comet.serde.QueryPlanSerde.{exprToProtoInternal, optExprWithInfo, scalarFunctionExprToProto}
@@ -29,17 +30,40 @@ import org.apache.comet.serde.QueryPlanSerde.{exprToProtoInternal, optExprWithIn
 // https://github.com/apache/datafusion/issues/16594
 object CometFromUnixTime extends CometExpressionSerde[FromUnixTime] {
 
+  override def getUnsupportedReasons(): Seq[String] = Seq(
+    "Only supports the default datetime format pattern `yyyy-MM-dd HH:mm:ss`")
+
   override def getIncompatibleReasons(): Seq[String] = Seq(
-    "Only supports the default datetime format pattern `yyyy-MM-dd HH:mm:ss`." +
-      " DataFusion's valid timestamp range differs from Spark" +
+    "DataFusion's valid timestamp range differs from Spark" +
       " (https://github.com/apache/datafusion/issues/16594)")
 
-  override def getSupportLevel(expr: FromUnixTime): SupportLevel = Incompatible(None)
+  override def getSupportLevel(expr: FromUnixTime): SupportLevel = {
+    expr.format match {
+      case Literal(null, _) =>
+        Compatible(None)
+      case Literal(fmt, _) =>
+        val formatStr = fmt.toString
+        val defaultPattern = TimestampFormatter.defaultPattern
+        if (formatStr == defaultPattern) {
+          Incompatible(None)
+        } else {
+          Unsupported(Some(s"Datetime pattern format: $formatStr is unsupported"))
+        }
+      case _ =>
+        Unsupported(Some("Non-literal datetime pattern format is unsupported"))
+    }
+  }
 
   override def convert(
       expr: FromUnixTime,
       inputs: Seq[Attribute],
       binding: Boolean): Option[ExprOuterClass.Expr] = {
+    expr.format match {
+      case Literal(null, _) =>
+        // from_unixtime is null-intolerant, so NULL format yields NULL
+        return exprToProtoInternal(Literal.create(null, StringType), inputs, binding)
+      case _ =>
+    }
     val secExpr = exprToProtoInternal(expr.sec, inputs, binding)
     // TODO: DataFusion toChar does not support Spark datetime pattern format
     // https://github.com/apache/datafusion/issues/16577
@@ -48,10 +72,7 @@ object CometFromUnixTime extends CometExpressionSerde[FromUnixTime] {
     val formatExpr = exprToProtoInternal(Literal("%Y-%m-%d %H:%M:%S"), inputs, binding)
     val timeZone = exprToProtoInternal(Literal(expr.timeZoneId.orNull), inputs, binding)
 
-    if (expr.format != Literal(TimestampFormatter.defaultPattern)) {
-      withInfo(expr, "Datetime pattern format is unsupported")
-      None
-    } else if (secExpr.isDefined && formatExpr.isDefined) {
+    if (secExpr.isDefined && formatExpr.isDefined) {
       val timestampExpr =
         scalarFunctionExprToProto("from_unixtime", Seq(secExpr, timeZone): _*)
       val optExpr = scalarFunctionExprToProto("to_char", Seq(timestampExpr, formatExpr): _*)

spark/src/test/resources/sql-tests/expressions/datetime/from_unix_time.sql

@@ -24,12 +24,22 @@ INSERT INTO test_from_unix_time VALUES (0), (1718451045), (-1), (NULL), (2147483
 query expect_fallback(not fully compatible with Spark)
 SELECT from_unixtime(t) FROM test_from_unix_time
 
-query expect_fallback(not fully compatible with Spark)
+query expect_fallback(Datetime pattern format: yyyy-MM-dd is unsupported)
 SELECT from_unixtime(t, 'yyyy-MM-dd') FROM test_from_unix_time
 
 -- literal arguments
 query expect_fallback(not fully compatible with Spark)
 SELECT from_unixtime(0)
 
-query expect_fallback(not fully compatible with Spark)
+query expect_fallback(Datetime pattern format: yyyy-MM-dd is unsupported)
 SELECT from_unixtime(1718451045, 'yyyy-MM-dd')
+
+-- null format literal
+query
+SELECT from_unixtime(t, NULL) FROM test_from_unix_time
+
+query
+SELECT from_unixtime(1718451045, NULL)
+
+query
+SELECT from_unixtime(1718451045, CAST(NULL AS STRING))

spark/src/test/resources/sql-tests/expressions/string/string_repeat.sql

@@ -35,3 +35,15 @@ SELECT repeat('hi', n) FROM test_repeat
 -- literal + literal
 query
 SELECT repeat('hi', 3), repeat('', 5), repeat('a', 0), repeat(NULL, 3)
+
+-- non-positive literal count
+query
+SELECT repeat('namaste', -1), repeat('namaste', -100), repeat('a', 0), repeat(NULL, -1)
+
+-- non-positive literal count over a column
+query
+SELECT repeat(s, -1), repeat(s, 0) FROM test_repeat
+
+-- non-literal count (column expression)
+query
+SELECT repeat(s, -n) FROM test_repeat