fix(udf): declare scalar UDF signatures with Field, not ArrowType

core/src/main/java/org/apache/datafusion/ScalarFunction.java

@@ -22,7 +22,7 @@
 import java.util.List;
 
 import org.apache.arrow.memory.BufferAllocator;
-import org.apache.arrow.vector.types.pojo.ArrowType;
+import org.apache.arrow.vector.types.pojo.Field;
 
 /**
  * A Java-implemented scalar SQL function. Implementations declare their own name, signature, and
@@ -40,15 +40,24 @@ public interface ScalarFunction {
   String name();
 
   /**
-   * Declared argument types, in positional order. The function is registered with an exact
+   * Declared argument fields, in positional order. The function is registered with an exact
    * signature; calls whose argument types do not match exactly are rejected.
+   *
+   * <p>Each entry is an Arrow {@link Field} -- a name plus a {@code FieldType} plus an optional
+   * list of child fields. Use {@link Field#nullable(String,
+   * org.apache.arrow.vector.types.pojo.ArrowType)} for primitive types (e.g. {@code
+   * Field.nullable("arg0", new ArrowType.Int(32, true))}). Nested types like {@code List}, {@code
+   * Struct}, and {@code Map} require the children list to carry element / member / key / value type
+   * information; constructing a {@code Field} via {@code new Field(name, FieldType, children)} is
+   * the canonical Arrow way to do that.
    */
-  List<ArrowType> argTypes();
+  List<Field> argFields();
 
   /**
-   * Declared return type. The returned {@link ColumnarValue}'s vector must have this exact type.
+   * Declared return field. The returned {@link ColumnarValue}'s vector must have this exact type,
+   * including any nested children. Same construction rules as {@link #argFields()}.
    */
-  ArrowType returnType();
+  Field returnField();
 
   /**
    * Volatility classification. Use {@link Volatility#IMMUTABLE} for pure functions, {@link

core/src/main/java/org/apache/datafusion/ScalarUdf.java

@@ -22,7 +22,7 @@
 import java.util.List;
 import java.util.Objects;
 
-import org.apache.arrow.vector.types.pojo.ArrowType;
+import org.apache.arrow.vector.types.pojo.Field;
 
 /**
  * A scalar UDF registration handle: pairs a {@link ScalarFunction} implementation with the metadata
@@ -35,8 +35,8 @@
 public final class ScalarUdf {
   private final ScalarFunction impl;
   private final String name;
-  private final List<ArrowType> argTypes;
-  private final ArrowType returnType;
+  private final List<Field> argFields;
+  private final Field returnField;
   private final Volatility volatility;
 
   /**
@@ -48,8 +48,8 @@ public final class ScalarUdf {
   public ScalarUdf(ScalarFunction impl) {
     this.impl = Objects.requireNonNull(impl, "impl");
     this.name = Objects.requireNonNull(impl.name(), "impl.name()");
-    this.argTypes = Objects.requireNonNull(impl.argTypes(), "impl.argTypes()");
-    this.returnType = Objects.requireNonNull(impl.returnType(), "impl.returnType()");
+    this.argFields = Objects.requireNonNull(impl.argFields(), "impl.argFields()");
+    this.returnField = Objects.requireNonNull(impl.returnField(), "impl.returnField()");
     this.volatility = Objects.requireNonNull(impl.volatility(), "impl.volatility()");
   }
 
@@ -68,14 +68,14 @@ public String name() {
     return name;
   }
 
-  /** Declared argument types; cached from {@link ScalarFunction#argTypes()}. */
-  public List<ArrowType> argTypes() {
-    return argTypes;
+  /** Declared argument fields; cached from {@link ScalarFunction#argFields()}. */
+  public List<Field> argFields() {
+    return argFields;
   }
 
-  /** Declared return type; cached from {@link ScalarFunction#returnType()}. */
-  public ArrowType returnType() {
-    return returnType;
+  /** Declared return field; cached from {@link ScalarFunction#returnField()}. */
+  public Field returnField() {
+    return returnField;
   }
 
   /** Volatility classification; cached from {@link ScalarFunction#volatility()}. */

core/src/main/java/org/apache/datafusion/SessionContext.java

@@ -32,9 +32,7 @@
 import org.apache.arrow.vector.ipc.ArrowStreamWriter;
 import org.apache.arrow.vector.ipc.ReadChannel;
 import org.apache.arrow.vector.ipc.message.MessageSerializer;
-import org.apache.arrow.vector.types.pojo.ArrowType;
 import org.apache.arrow.vector.types.pojo.Field;
-import org.apache.arrow.vector.types.pojo.FieldType;
 import org.apache.arrow.vector.types.pojo.Schema;
 
 /**
@@ -367,7 +365,7 @@ public DataFrame readArrow(String path, ArrowReadOptions options) {
    * via the UDF's name or referenced in DataFusion plans deserialised with {@link #fromProto}.
    *
    * <p>The UDF is registered with an exact signature: the runtime will reject calls whose argument
-   * types do not match the declared {@link ScalarFunction#argTypes()} exactly.
+   * types do not match the declared {@link ScalarFunction#argFields()} exactly.
    *
    * @throws RuntimeException if registration fails (e.g., name already registered with an
    *     incompatible signature, schema serialisation failure).
@@ -379,14 +377,10 @@ public void registerUdf(ScalarUdf udf) {
     java.util.Objects.requireNonNull(udf, "udf");
     ScalarFunction impl = udf.impl();
     String name = udf.name();
-    ArrowType returnType = udf.returnType();
-    List<ArrowType> argTypes = udf.argTypes();
     Volatility volatility = udf.volatility();
-    List<Field> fields = new ArrayList<>(argTypes.size() + 1);
-    fields.add(new Field("return", FieldType.nullable(returnType), null));
-    for (int i = 0; i < argTypes.size(); i++) {
-      fields.add(new Field("arg" + i, FieldType.nullable(argTypes.get(i)), null));
-    }
+    List<Field> fields = new ArrayList<>(udf.argFields().size() + 1);
+    fields.add(udf.returnField());
+    fields.addAll(udf.argFields());
     Schema signatureSchema = new Schema(fields);
     byte[] signatureBytes = serializeSchemaIpc(signatureSchema);
     registerScalarUdf(nativeHandle, name, signatureBytes, volatility.code(), impl);