Allow Constructing Non-Empty StructArray with no Fields (#4842) (#4845)

arrow-array/src/array/struct_array.rs

@@ -197,6 +197,23 @@ impl StructArray {
         }
     }
 
+    /// Create a new [`StructArray`] containing no fields
+    ///
+    /// # Panics
+    ///
+    /// If `len != nulls.len()`
+    pub fn new_empty_fields(len: usize, nulls: Option<NullBuffer>) -> Self {
+        if let Some(n) = &nulls {
+            assert_eq!(len, n.len())
+        }
+        Self {
+            len,
+            data_type: DataType::Struct(Fields::empty()),
+            fields: vec![],
+            nulls,
+        }
+    }
+
     /// Deconstruct this array into its constituent parts
     pub fn into_parts(self) -> (Fields, Vec<ArrayRef>, Option<NullBuffer>) {
         let f = match self.data_type {

arrow-array/src/builder/struct_builder.rs

@@ -233,6 +233,12 @@ impl StructBuilder {
     /// Builds the `StructArray` and reset this builder.
     pub fn finish(&mut self) -> StructArray {
         self.validate_content();
+        if self.fields.is_empty() {
+            return StructArray::new_empty_fields(
+                self.len(),
+                self.null_buffer_builder.finish(),
+            );
+        }
 
         let arrays = self.field_builders.iter_mut().map(|f| f.finish()).collect();
         let nulls = self.null_buffer_builder.finish();
@@ -243,6 +249,13 @@ impl StructBuilder {
     pub fn finish_cloned(&self) -> StructArray {
         self.validate_content();
 
+        if self.fields.is_empty() {
+            return StructArray::new_empty_fields(
+                self.len(),
+                self.null_buffer_builder.finish_cloned(),
+            );
+        }
+
         let arrays = self
             .field_builders
             .iter()
@@ -591,4 +604,19 @@ mod tests {
         let mut sa = StructBuilder::new(fields, field_builders);
         sa.finish();
     }
+
+    #[test]
+    fn test_empty() {
+        let mut builder = StructBuilder::new(Fields::empty(), vec![]);
+        builder.append(true);
+        builder.append(false);
+
+        let a1 = builder.finish_cloned();
+        let a2 = builder.finish();
+        assert_eq!(a1, a2);
+        assert_eq!(a1.len(), 2);
+        assert_eq!(a1.null_count(), 1);
+        assert!(a1.is_valid(0));
+        assert!(a1.is_null(1));
+    }
 }