andygrove commented on code in PR #4111:
URL: https://github.com/apache/datafusion-comet/pull/4111#discussion_r3289680116


##########
spark/src/main/scala/org/apache/comet/serde/strings.scala:
##########
@@ -33,15 +33,21 @@ 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],
       binding: Boolean): Option[ExprOuterClass.Expr] = {
     val children = expr.children
+    children(1) match {
+      case Literal(count: Number, _) if count.longValue() <= 0 =>
+        // Match Spark: repeat(s, n) with n <= 0 returns "" (or NULL if s is 
NULL)
+        val ifExpr = If(
+          IsNull(children(0)),
+          Literal.create(null, StringType),
+          Literal(UTF8String.EMPTY_UTF8, StringType))
+        return exprToProtoInternal(ifExpr, inputs, binding)
+      case _ =>

Review Comment:
   we could consider just falling back to Spark if the second arg is not a 
literal



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