arrow-select: add support for merging primitive dictionary values

Previously, should_merge_dictionaries would always return false in the ptr_eq
closure creation match arm for types that were not {Large}{Utf8,Binary}. This
could lead to excessive memory usage.

Author: asubiotto

arrow-select/src/concat.rs

@@ -1010,6 +1010,43 @@ mod tests {
         assert!((30..40).contains(&values_len), "{values_len}")
     }
 
+    #[test]
+    fn test_primitive_dictionary_merge() {
+        // Same value repeated 5 times.
+        let keys = vec![1; 5];
+        let values = (10..20).collect::<Vec<_>>();
+        let dict = DictionaryArray::new(
+            Int8Array::from(keys.clone()),
+            Arc::new(Int32Array::from(values.clone())),
+        );
+        let other = DictionaryArray::new(
+            Int8Array::from(keys.clone()),
+            Arc::new(Int32Array::from(values.clone())),
+        );
+
+        let result_same_dictionary = concat(&[&dict, &dict]).unwrap();
+        // Verify pointer equality check succeeds, and therefore the
+        // dictionaries are not merged. A single values buffer should be reused
+        // in this case.
+        assert_eq!(
+            result_same_dictionary
+                .as_dictionary::<Int8Type>()
+                .values()
+                .len(),
+            values.len(),
+        );
+
+        let result_cloned_dictionary = concat(&[&dict, &other]).unwrap();
+        // Should have only 1 underlying value since all keys reference it.
+        assert_eq!(
+            result_cloned_dictionary
+                .as_dictionary::<Int8Type>()
+                .values()
+                .len(),
+            1
+        );
+    }
+
     #[test]
     fn test_concat_string_sizes() {
         let a: LargeStringArray = ((0..150).map(|_| Some("foo"))).collect();

arrow-select/src/dictionary.rs

@@ -18,12 +18,13 @@
 use crate::interleave::interleave;
 use ahash::RandomState;
 use arrow_array::builder::BooleanBufferBuilder;
-use arrow_array::cast::AsArray;
 use arrow_array::types::{
-    ArrowDictionaryKeyType, BinaryType, ByteArrayType, LargeBinaryType, LargeUtf8Type, Utf8Type,
+    ArrowDictionaryKeyType, ArrowPrimitiveType, BinaryType, ByteArrayType, LargeBinaryType,
+    LargeUtf8Type, Utf8Type,
 };
-use arrow_array::{Array, ArrayRef, DictionaryArray, GenericByteArray};
-use arrow_buffer::{ArrowNativeType, BooleanBuffer, ScalarBuffer};
+use arrow_array::{cast::AsArray, downcast_primitive};
+use arrow_array::{Array, ArrayRef, DictionaryArray, GenericByteArray, PrimitiveArray};
+use arrow_buffer::{ArrowNativeType, BooleanBuffer, ScalarBuffer, ToByteSlice};
 use arrow_schema::{ArrowError, DataType};
 
 /// A best effort interner that maintains a fixed number of buckets
@@ -120,7 +121,12 @@ pub fn should_merge_dictionary_values<K: ArrowDictionaryKeyType>(
         LargeUtf8 => Box::new(bytes_ptr_eq::<LargeUtf8Type>),
         Binary => Box::new(bytes_ptr_eq::<BinaryType>),
         LargeBinary => Box::new(bytes_ptr_eq::<LargeBinaryType>),
-        _ => return false,
+        dt => {
+            if !dt.is_primitive() {
+                return false;
+            }
+            Box::new(|a, b| a.to_data().ptr_eq(&b.to_data()))
+        }
     };
 
     let mut single_dictionary = true;
@@ -233,17 +239,43 @@ fn compute_values_mask<K: ArrowNativeType>(
     builder.finish()
 }
 
+/// Process primitive array values to bytes
+fn masked_primitives_to_bytes<'a, T: ArrowPrimitiveType>(
+    array: &'a PrimitiveArray<T>,
+    mask: &BooleanBuffer,
+) -> Vec<(usize, Option<&'a [u8]>)>
+where
+    T::Native: ToByteSlice,
+{
+    let mut out = Vec::with_capacity(mask.count_set_bits());
+    let values = array.values();
+    for idx in mask.set_indices() {
+        out.push((
+            idx,
+            array.is_valid(idx).then_some(values[idx].to_byte_slice()),
+        ))
+    }
+    out
+}
+
+macro_rules! masked_primitive_to_bytes_helper {
+    ($t:ty, $array:expr, $mask:expr) => {
+        masked_primitives_to_bytes::<$t>($array.as_primitive(), $mask)
+    };
+}
+
 /// Return a Vec containing for each set index in `mask`, the index and byte value of that index
 fn get_masked_values<'a>(
     array: &'a dyn Array,
     mask: &BooleanBuffer,
 ) -> Vec<(usize, Option<&'a [u8]>)> {
-    match array.data_type() {
+    downcast_primitive! {
+        array.data_type() => (masked_primitive_to_bytes_helper, array, mask),
         DataType::Utf8 => masked_bytes(array.as_string::<i32>(), mask),
         DataType::LargeUtf8 => masked_bytes(array.as_string::<i64>(), mask),
         DataType::Binary => masked_bytes(array.as_binary::<i32>(), mask),
         DataType::LargeBinary => masked_bytes(array.as_binary::<i64>(), mask),
-        _ => unimplemented!(),
+        _ => unimplemented!("Dictionary merging for type {} is not implemented", array.data_type()),
     }
 }