perf: optimize spark_hex dictionary path by avoiding dictionary expansion (#19832)

## Which issue does this PR close?

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Follow up to
#19738

## Rationale for this change

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in the issue then this section is not needed.
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your changes and offer better suggestions for fixes.
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The current hex implementation expands `DictionaryArray` inputs into a
regular array, which causes loss of dictionary encoding and redundant
hex computation for repeated values.

## What changes are included in this PR?

- Apply hex encoding only to dictionary values 
- Avoid expanding dictionary arrays during execution

### Benchmark
| Size | Before | After | Speedup |
| ---- | ------ | ----- | ------- |
| 1024 | 8.3 µs | 7.2 µs | 1.15× |
| 4096 | 42.9 µs | 34.5 µs | 1.24× |
| 8192 | 91.6 µs | 71.7 µs | 1.28× |

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## Are these changes tested?
Yes. Existing unit tests and `sqllogictest` tests pass.

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## Are there any user-facing changes?
No.

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Author: lyne7-sc

datafusion/spark/src/function/math/hex.rs

@@ -19,7 +19,7 @@ use std::any::Any;
 use std::str::from_utf8_unchecked;
 use std::sync::Arc;
 
-use arrow::array::{Array, BinaryArray, Int64Array, StringArray, StringBuilder};
+use arrow::array::{Array, ArrayRef, StringBuilder};
 use arrow::datatypes::DataType;
 use arrow::{
     array::{as_dictionary_array, as_largestring_array, as_string_array},
@@ -92,11 +92,13 @@ impl ScalarUDFImpl for SparkHex {
         &self.signature
     }
 
-    fn return_type(
-        &self,
-        _arg_types: &[DataType],
-    ) -> datafusion_common::Result<DataType> {
-        Ok(DataType::Utf8)
+    fn return_type(&self, arg_types: &[DataType]) -> datafusion_common::Result<DataType> {
+        Ok(match &arg_types[0] {
+            DataType::Dictionary(key_type, _) => {
+                DataType::Dictionary(key_type.clone(), Box::new(DataType::Utf8))
+            }
+            _ => DataType::Utf8,
+        })
     }
 
     fn invoke_with_args(
@@ -136,7 +138,7 @@ fn hex_encode_bytes<'a, I, T>(
     iter: I,
     lowercase: bool,
     len: usize,
-) -> Result<ColumnarValue, DataFusionError>
+) -> Result<ArrayRef, DataFusionError>
 where
     I: Iterator<Item = Option<T>>,
     T: AsRef<[u8]> + 'a,
@@ -166,14 +168,14 @@ where
         }
     }
 
-    Ok(ColumnarValue::Array(Arc::new(builder.finish())))
+    Ok(Arc::new(builder.finish()))
 }
 
 /// Generic hex encoding for int64 type
-fn hex_encode_int64<I>(iter: I, len: usize) -> Result<ColumnarValue, DataFusionError>
-where
-    I: Iterator<Item = Option<i64>>,
-{
+fn hex_encode_int64(
+    iter: impl Iterator<Item = Option<i64>>,
+    len: usize,
+) -> Result<ArrayRef, DataFusionError> {
     let mut builder = StringBuilder::with_capacity(len, len * 16);
 
     for v in iter {
@@ -189,7 +191,7 @@ where
         }
     }
 
-    Ok(ColumnarValue::Array(Arc::new(builder.finish())))
+    Ok(Arc::new(builder.finish()))
 }
 
 /// Spark-compatible `hex` function
@@ -215,55 +217,109 @@ pub fn compute_hex(
         ColumnarValue::Array(array) => match array.data_type() {
             DataType::Int64 => {
                 let array = as_int64_array(array)?;
-                hex_encode_int64(array.iter(), array.len())
+                Ok(ColumnarValue::Array(hex_encode_int64(
+                    array.iter(),
+                    array.len(),
+                )?))
             }
             DataType::Utf8 => {
                 let array = as_string_array(array);
-                hex_encode_bytes(array.iter(), lowercase, array.len())
+                Ok(ColumnarValue::Array(hex_encode_bytes(
+                    array.iter(),
+                    lowercase,
+                    array.len(),
+                )?))
             }
             DataType::Utf8View => {
                 let array = as_string_view_array(array)?;
-                hex_encode_bytes(array.iter(), lowercase, array.len())
+                Ok(ColumnarValue::Array(hex_encode_bytes(
+                    array.iter(),
+                    lowercase,
+                    array.len(),
+                )?))
             }
             DataType::LargeUtf8 => {
                 let array = as_largestring_array(array);
-                hex_encode_bytes(array.iter(), lowercase, array.len())
+                Ok(ColumnarValue::Array(hex_encode_bytes(
+                    array.iter(),
+                    lowercase,
+                    array.len(),
+                )?))
             }
             DataType::Binary => {
                 let array = as_binary_array(array)?;
-                hex_encode_bytes(array.iter(), lowercase, array.len())
+                Ok(ColumnarValue::Array(hex_encode_bytes(
+                    array.iter(),
+                    lowercase,
+                    array.len(),
+                )?))
             }
             DataType::LargeBinary => {
                 let array = as_large_binary_array(array)?;
-                hex_encode_bytes(array.iter(), lowercase, array.len())
+                Ok(ColumnarValue::Array(hex_encode_bytes(
+                    array.iter(),
+                    lowercase,
+                    array.len(),
+                )?))
             }
             DataType::FixedSizeBinary(_) => {
                 let array = as_fixed_size_binary_array(array)?;
-                hex_encode_bytes(array.iter(), lowercase, array.len())
+                Ok(ColumnarValue::Array(hex_encode_bytes(
+                    array.iter(),
+                    lowercase,
+                    array.len(),
+                )?))
             }
-            DataType::Dictionary(_, value_type) => {
+            DataType::Dictionary(key_type, _) => {
+                if **key_type != DataType::Int32 {
+                    return exec_err!(
+                        "hex only supports Int32 dictionary keys, get: {}",
+                        key_type
+                    );
+                }
+
                 let dict = as_dictionary_array::<Int32Type>(&array);
+                let dict_values = dict.values();
 
-                match **value_type {
+                let encoded_values = match dict_values.data_type() {
                     DataType::Int64 => {
-                        let arr = dict.downcast_dict::<Int64Array>().unwrap();
-                        hex_encode_int64(arr.into_iter(), dict.len())
+                        let arr = as_int64_array(dict_values)?;
+                        hex_encode_int64(arr.iter(), arr.len())?
                     }
                     DataType::Utf8 => {
-                        let arr = dict.downcast_dict::<StringArray>().unwrap();
-                        hex_encode_bytes(arr.into_iter(), lowercase, dict.len())
+                        let arr = as_string_array(dict_values);
+                        hex_encode_bytes(arr.iter(), lowercase, arr.len())?
+                    }
+                    DataType::LargeUtf8 => {
+                        let arr = as_largestring_array(dict_values);
+                        hex_encode_bytes(arr.iter(), lowercase, arr.len())?
+                    }
+                    DataType::Utf8View => {
+                        let arr = as_string_view_array(dict_values)?;
+                        hex_encode_bytes(arr.iter(), lowercase, arr.len())?
                     }
                     DataType::Binary => {
-                        let arr = dict.downcast_dict::<BinaryArray>().unwrap();
-                        hex_encode_bytes(arr.into_iter(), lowercase, dict.len())
+                        let arr = as_binary_array(dict_values)?;
+                        hex_encode_bytes(arr.iter(), lowercase, arr.len())?
+                    }
+                    DataType::LargeBinary => {
+                        let arr = as_large_binary_array(dict_values)?;
+                        hex_encode_bytes(arr.iter(), lowercase, arr.len())?
+                    }
+                    DataType::FixedSizeBinary(_) => {
+                        let arr = as_fixed_size_binary_array(dict_values)?;
+                        hex_encode_bytes(arr.iter(), lowercase, arr.len())?
                     }
                     _ => {
-                        exec_err!(
+                        return exec_err!(
                             "hex got an unexpected argument type: {}",
-                            array.data_type()
-                        )
+                            dict_values.data_type()
+                        );
                     }
-                }
+                };
+
+                let new_dict = dict.with_values(encoded_values);
+                Ok(ColumnarValue::Array(Arc::new(new_dict)))
             }
             _ => exec_err!("hex got an unexpected argument type: {}", array.data_type()),
         },
@@ -279,11 +335,12 @@ mod test {
     use arrow::array::{DictionaryArray, Int32Array, Int64Array, StringArray};
     use arrow::{
         array::{
-            BinaryDictionaryBuilder, PrimitiveDictionaryBuilder, StringBuilder,
-            StringDictionaryBuilder, as_string_array,
+            BinaryDictionaryBuilder, PrimitiveDictionaryBuilder, StringDictionaryBuilder,
+            as_string_array,
         },
         datatypes::{Int32Type, Int64Type},
     };
+    use datafusion_common::cast::as_dictionary_array;
     use datafusion_expr::ColumnarValue;
 
     #[test]
@@ -295,12 +352,12 @@ mod test {
         input_builder.append_value("rust");
         let input = input_builder.finish();
 
-        let mut string_builder = StringBuilder::new();
-        string_builder.append_value("6869");
-        string_builder.append_value("627965");
-        string_builder.append_null();
-        string_builder.append_value("72757374");
-        let expected = string_builder.finish();
+        let mut expected_builder = StringDictionaryBuilder::<Int32Type>::new();
+        expected_builder.append_value("6869");
+        expected_builder.append_value("627965");
+        expected_builder.append_null();
+        expected_builder.append_value("72757374");
+        let expected = expected_builder.finish();
 
         let columnar_value = ColumnarValue::Array(Arc::new(input));
         let result = super::spark_hex(&[columnar_value]).unwrap();
@@ -310,7 +367,7 @@ mod test {
             _ => panic!("Expected array"),
         };
 
-        let result = as_string_array(&result);
+        let result = as_dictionary_array(&result).unwrap();
 
         assert_eq!(result, &expected);
     }
@@ -324,12 +381,12 @@ mod test {
         input_builder.append_value(3);
         let input = input_builder.finish();
 
-        let mut string_builder = StringBuilder::new();
-        string_builder.append_value("1");
-        string_builder.append_value("2");
-        string_builder.append_null();
-        string_builder.append_value("3");
-        let expected = string_builder.finish();
+        let mut expected_builder = StringDictionaryBuilder::<Int32Type>::new();
+        expected_builder.append_value("1");
+        expected_builder.append_value("2");
+        expected_builder.append_null();
+        expected_builder.append_value("3");
+        let expected = expected_builder.finish();
 
         let columnar_value = ColumnarValue::Array(Arc::new(input));
         let result = super::spark_hex(&[columnar_value]).unwrap();
@@ -339,7 +396,7 @@ mod test {
             _ => panic!("Expected array"),
         };
 
-        let result = as_string_array(&result);
+        let result = as_dictionary_array(&result).unwrap();
 
         assert_eq!(result, &expected);
     }
@@ -353,7 +410,7 @@ mod test {
         input_builder.append_value("3");
         let input = input_builder.finish();
 
-        let mut expected_builder = StringBuilder::new();
+        let mut expected_builder = StringDictionaryBuilder::<Int32Type>::new();
         expected_builder.append_value("31");
         expected_builder.append_value("6A");
         expected_builder.append_null();
@@ -368,7 +425,7 @@ mod test {
             _ => panic!("Expected array"),
         };
 
-        let result = as_string_array(&result);
+        let result = as_dictionary_array(&result).unwrap();
 
         assert_eq!(result, &expected);
     }
@@ -425,8 +482,11 @@ mod test {
             _ => panic!("Expected array"),
         };
 
-        let result = as_string_array(&result);
-        let expected = StringArray::from(vec![Some("20"), None, None]);
+        let result = as_dictionary_array(&result).unwrap();
+
+        let keys = Int32Array::from(vec![Some(0), None, Some(1)]);
+        let vals = StringArray::from(vec![Some("20"), None]);
+        let expected = DictionaryArray::new(keys, Arc::new(vals));
 
         assert_eq!(&expected, result);
     }

datafusion/sqllogictest/test_files/spark/math/hex.slt

@@ -63,3 +63,23 @@ query T
 SELECT hex(arrow_cast('test', 'LargeBinary')) as lar_b;
 ----
 74657374
+
+statement ok
+CREATE TABLE t_dict_binary AS
+SELECT arrow_cast(column1, 'Dictionary(Int32, Binary)') as dict_col
+FROM VALUES ('foo'), ('bar'), ('foo'), (NULL), ('baz'), ('bar');
+
+query T
+SELECT hex(dict_col) FROM t_dict_binary;
+----
+666F6F
+626172
+666F6F
+NULL
+62617A
+626172
+
+query T
+SELECT arrow_typeof(hex(dict_col)) FROM t_dict_binary LIMIT 1;
+----
+Dictionary(Int32, Utf8)