duckdb · Maxxen · Feb 24, 2025 · Feb 18, 2025 · Feb 23, 2025 · Feb 24, 2025
diff --git a/crates/duckdb/src/core/vector.rs b/crates/duckdb/src/core/vector.rs
@@ -192,7 +192,8 @@ impl ListVector {
     }
 
     /// Take the child as [StructVector].
-    pub fn struct_child(&self) -> StructVector {
+    pub fn struct_child(&self, capacity: usize) -> StructVector {
+        self.reserve(capacity);
         StructVector::from(unsafe { duckdb_list_vector_get_child(self.entries.ptr) })
     }
 
@@ -300,8 +301,11 @@ impl From<duckdb_vector> for StructVector {
 
 impl StructVector {
     /// Returns the child by idx in the list vector.
-    pub fn child(&self, idx: usize) -> FlatVector {
-        FlatVector::from(unsafe { duckdb_struct_vector_get_child(self.ptr, idx as u64) })
+    pub fn child(&self, idx: usize, capacity: usize) -> FlatVector {
+        FlatVector::with_capacity(
+            unsafe { duckdb_struct_vector_get_child(self.ptr, idx as u64) },
+            capacity,
+        )
     }
 
     /// Take the child as [StructVector].

diff --git a/crates/duckdb/src/vtab/arrow.rs b/crates/duckdb/src/vtab/arrow.rs
@@ -6,6 +6,7 @@ use crate::{
     core::{ArrayVector, FlatVector, Inserter, ListVector, StructVector, Vector},
     types::DuckString,
 };
+use arrow::array::as_map_array;
 use arrow::{
     array::{
         as_boolean_array, as_generic_binary_array, as_large_list_array, as_list_array, as_primitive_array,
@@ -204,6 +205,7 @@ pub fn to_duckdb_logical_type(data_type: &DataType) -> Result<LogicalTypeHandle,
             // DuckDB does not support negative decimal scales
             Ok(LogicalTypeHandle::decimal(*width, (*scale).try_into().unwrap()))
         }
+        DataType::Map(field, _) => arrow_map_to_duckdb_logical_type(field),
         DataType::Boolean
         | DataType::Utf8
         | DataType::LargeUtf8
@@ -220,6 +222,35 @@ pub fn to_duckdb_logical_type(data_type: &DataType) -> Result<LogicalTypeHandle,
     }
 }
 
+fn arrow_map_to_duckdb_logical_type(field: &FieldRef) -> Result<LogicalTypeHandle, Box<dyn std::error::Error>> {
+    // Map is a logical nested type that is represented as `List<entries: Struct<key: K, value: V>>`
+    let DataType::Struct(ref fields) = field.data_type() else {
+        return Err(format!(
+            "The inner field of a Map must be a Struct, got: {:?}",
+            field.data_type()
+        )
+        .into());
+    };
+
+    if fields.len() != 2 {
+        return Err(format!(
+            "The inner Struct field of a Map must have 2 fields, got {} fields",
+            fields.len()
+        )
+        .into());
+    }
+
+    let (Some(key_field), Some(value_field)) = (fields.first(), fields.get(1)) else {
+        // number of fields is verified above
+        unreachable!()
+    };
+
+    Ok(LogicalTypeHandle::map(
+        &LogicalTypeHandle::from(to_duckdb_type_id(key_field.data_type())?),
+        &LogicalTypeHandle::from(to_duckdb_type_id(value_field.data_type())?),
+    ))
+}
+
 // FIXME: flat vectors don't have all of thsese types. I think they only
 /// Converts flat vector to an arrow array
 pub fn flat_vector_to_arrow_array(
@@ -586,6 +617,19 @@ pub fn write_arrow_array_to_vector(
             let mut struct_vector = chunk.struct_vector();
             struct_array_to_vector(struct_array, &mut struct_vector)?;
         }
+        DataType::Map(_, _) => {
+            // [`MapArray`] is physically a [`ListArray`] of key values pairs stored as an `entries` [`StructArray`] with 2 child fields.
+            let map_array = as_map_array(col.as_ref());
+            let out = &mut chunk.list_vector();
+            struct_array_to_vector(map_array.entries(), &mut out.struct_child(map_array.entries().len()))?;
+
+            for i in 0..map_array.len() {
+                let offset = map_array.value_offsets()[i];
+                let length = map_array.value_length(i);
+                out.set_entry(i, offset.as_(), length.as_());
+            }
+            set_nulls_in_list_vector(map_array, out);
+        }
         dt => {
             return Err(format!(
                 "column with data_type {} is not supported yet, please file an issue https://github.com/wangfenjin/duckdb-rs",
@@ -935,7 +979,10 @@ fn list_array_to_vector<O: OffsetSizeTrait + AsPrimitive<usize>>(
             fixed_size_list_array_to_vector(as_fixed_size_list_array(value_array.as_ref()), &mut out.array_child())?;
         }
         DataType::Struct(_) => {
-            struct_array_to_vector(as_struct_array(value_array.as_ref()), &mut out.struct_child())?;
+            struct_array_to_vector(
+                as_struct_array(value_array.as_ref()),
+                &mut out.struct_child(value_array.len()),
+            )?;
         }
         _ => {
             return Err(format!(
@@ -993,13 +1040,13 @@ fn struct_array_to_vector(array: &StructArray, out: &mut StructVector) -> Result
         let column = array.column(i);
         match column.data_type() {
             dt if dt.is_primitive() || matches!(dt, DataType::Boolean) => {
-                primitive_array_to_vector(column, &mut out.child(i))?;
+                primitive_array_to_vector(column, &mut out.child(i, array.len()))?;
             }
             DataType::Utf8 => {
-                string_array_to_vector(as_string_array(column.as_ref()), &mut out.child(i));
+                string_array_to_vector(as_string_array(column.as_ref()), &mut out.child(i, array.len()));
             }
             DataType::Binary => {
-                binary_array_to_vector(as_generic_binary_array(column.as_ref()), &mut out.child(i));
+                binary_array_to_vector(as_generic_binary_array(column.as_ref()), &mut out.child(i, array.len()));
             }
             DataType::List(_) => {
                 list_array_to_vector(as_list_array(column.as_ref()), &mut out.list_vector_child(i))?;
@@ -1112,10 +1159,10 @@ mod test {
             Array, ArrayRef, AsArray, BinaryArray, BinaryViewArray, BooleanArray, Date32Array, Date64Array,
             Decimal128Array, Decimal256Array, DurationSecondArray, FixedSizeListArray, FixedSizeListBuilder,
             GenericByteArray, GenericListArray, Int32Array, Int32Builder, IntervalDayTimeArray,
-            IntervalMonthDayNanoArray, IntervalYearMonthArray, LargeStringArray, ListArray, ListBuilder,
+            IntervalMonthDayNanoArray, IntervalYearMonthArray, LargeStringArray, ListArray, ListBuilder, MapArray,
             OffsetSizeTrait, PrimitiveArray, StringArray, StringViewArray, StructArray, Time32SecondArray,
             Time64MicrosecondArray, TimestampMicrosecondArray, TimestampMillisecondArray, TimestampNanosecondArray,
-            TimestampSecondArray,
+            TimestampSecondArray, UInt32Array,
         },
         buffer::{OffsetBuffer, ScalarBuffer},
         d
8000
atatypes::{
@@ -1894,4 +1941,66 @@ mod test {
 
         Ok(())
     }
+
+    fn check_map_array_roundtrip(array: MapArray) -> Result<(), Box<dyn Error>> {
+        let expected = array.clone();
+
+        let db = Connection::open_in_memory()?;
+        db.register_table_function::<ArrowVTab>("arrow")?;
+
+        // Roundtrip a record batch from Rust to DuckDB and back to Rust
+        let schema = Schema::new(vec![Field::new("a", array.data_type().clone(), true)]);
+
+        let rb = RecordBatch::try_new(Arc::new(schema), vec![Arc::new(array.clone())])?;
+        let param = arrow_recordbatch_to_query_params(rb.clone());
+        let mut stmt = db.prepare("select a from arrow(?, ?)")?;
+        let rb = stmt.query_arrow(param)?.next().expect("no record batch");
+        let output_array = rb
+            .column(0)
+            .as_any()
+            .downcast_ref::<MapArray>()
+            .expect("Expected MapArray");
+
+        assert_eq!(output_array.keys(), expected.keys());
+        assert_eq!(output_array.values(), expected.values());
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_map_roundtrip() -> Result<(), Box<dyn Error>> {
+        // Test 1 - simple MapArray
+        let keys = vec!["a", "b", "c", "d", "e", "f", "g", "h"];
+        let values_data = UInt32Array::from(vec![
+            Some(0u32),
+            None,
+            Some(20),
+            Some(30),
+            None,
+            Some(50),
+            Some(60),
+            Some(70),
+        ]);
+        // Construct a buffer for value offsets, for the nested array:
+        //  [[a, b, c], [d, e, f], [g, h]]
+        let entry_offsets = [0, 3, 6, 8];
+        let map_array = MapArray::new_from_strings(keys.clone().into_iter(), &values_data, &entry_offsets).unwrap();
+        check_map_array_roundtrip(map_array)?;
+
+        // Test 2 - large MapArray of 4000 elements to test buffers capacity adjustment
+        let keys: Vec<String> = (0..
63CC
4000).map(|i| format!("key-{}", i)).collect();
+        let values_data = UInt32Array::from(
+            (0..4000)
+                .map(|i| if i % 5 == 0 { None } else { Some(i as u32) })
+                .collect::<Vec<_>>(),
+        );
+        let mut entry_offsets: Vec<u32> = (0..=4000).step_by(3).collect();
+        entry_offsets.push(4000);
+        let map_array =
+            MapArray::new_from_strings(keys.iter().map(String::as_str), &values_data, entry_offsets.as_slice())
+                .unwrap();
+        check_map_array_roundtrip(map_array)?;
+
+        Ok(())
+    }
 }