perf: Optimize scalar fast path & write() encoding for sha2 (#20116)

## Which issue does this PR close?

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- Closes #20046 .

## Rationale for this change

Spark `sha2` currently evaluates scalars via
`make_scalar_function(sha2_impl, vec![])`, which expands scalar inputs
to size-1 arrays before execution. This adds avoidable overhead for
scalar evaluation / constant folding scenarios.

In addition, the existing digest-to-hex formatting uses `write!(&mut s,
"{b:02x}")` in a loop, which is significantly slower than a LUT-based
hex encoder.

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your changes and offer better suggestions for fixes.
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## What changes are included in this PR?

1) a match-based scalar fast path for `sha2` to avoid scalar→array
expansion, and
2) a faster LUT-based hex encoder to replace `write!` formatting.

| Benchmark | Before | After | Speedup |
|----------|--------|-------|---------|
| `sha2/scalar/size=1` | 1.0408 µs | 339.29 ns | **~3.07x** |
| `sha2/array_binary_256/size=1024` | 604.13 µs | 295.09 µs | **~2.05x**
|
| `sha2/array_binary_256/size=4096` | 2.3508 ms | 1.2095 ms | **~1.94x**
|
| `sha2/array_binary_256/size=8192` | 4.5192 ms | 2.2826 ms | **~1.98x**
|

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## Are these changes tested?

Yes

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## Are there any user-facing changes?

No

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---------

Co-authored-by: Jeffrey Vo <jeffrey.vo.australia@gmail.com>
Co-authored-by: Martin Grigorov <martin-g@users.noreply.github.com>
Co-authored-by: Oleks V <comphead@users.noreply.github.com>

Author: 4 people

datafusion/spark/Cargo.toml

@@ -92,3 +92,7 @@ name = "substring"
 [[bench]]
 harness = false
 name = "unhex"
+
+[[bench]]
+harness = false
+name = "sha2"

datafusion/spark/benches/sha2.rs

@@ -0,0 +1,107 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+extern crate criterion;
+
+use arrow::array::*;
+use arrow::datatypes::*;
+use criterion::{Criterion, criterion_group, criterion_main};
+use datafusion_common::ScalarValue;
+use datafusion_common::config::ConfigOptions;
+use datafusion_expr::{ColumnarValue, ScalarFunctionArgs, ScalarUDFImpl};
+use datafusion_spark::function::hash::sha2::SparkSha2;
+use rand::rngs::StdRng;
+use rand::{Rng, SeedableRng};
+use std::hint::black_box;
+use std::sync::Arc;
+
+fn seedable_rng() -> StdRng {
+    StdRng::seed_from_u64(42)
+}
+
+fn generate_binary_data(size: usize, null_density: f32) -> BinaryArray {
+    let mut rng = seedable_rng();
+    let mut builder = BinaryBuilder::new();
+    for _ in 0..size {
+        if rng.random::<f32>() < null_density {
+            builder.append_null();
+        } else {
+            let len = rng.random_range::<usize, _>(1..=100);
+            let bytes: Vec<u8> = (0..len).map(|_| rng.random()).collect();
+            builder.append_value(&bytes);
+        }
+    }
+    builder.finish()
+}
+
+fn run_benchmark(c: &mut Criterion, name: &str, size: usize, args: &[ColumnarValue]) {
+    let sha2_func = SparkSha2::new();
+    let arg_fields: Vec<_> = args
+        .iter()
+        .enumerate()
+        .map(|(idx, arg)| Field::new(format!("arg_{idx}"), arg.data_type(), true).into())
+        .collect();
+    let config_options = Arc::new(ConfigOptions::default());
+
+    c.bench_function(&format!("{name}/size={size}"), |b| {
+        b.iter(|| {
+            black_box(
+                sha2_func
+                    .invoke_with_args(ScalarFunctionArgs {
+                        args: args.to_vec(),
+                        arg_fields: arg_fields.clone(),
+                        number_rows: size,
+                        return_field: Arc::new(Field::new("f", DataType::Utf8, true)),
+                        config_options: Arc::clone(&config_options),
+                    })
+                    .unwrap(),
+            )
+        })
+    });
+}
+
+fn criterion_benchmark(c: &mut Criterion) {
+    // Scalar benchmark (avoid array expansion)
+    let scalar_args = vec![
+        ColumnarValue::Scalar(ScalarValue::Binary(Some(b"Spark".to_vec()))),
+        ColumnarValue::Scalar(ScalarValue::Int32(Some(256))),
+    ];
+    run_benchmark(c, "sha2/scalar", 1, &scalar_args);
+
+    let sizes = vec![1024, 4096, 8192];
+    let null_density = 0.1;
+
+    for &size in &sizes {
+        let values: ArrayRef = Arc::new(generate_binary_data(size, null_density));
+        let bit_lengths: ArrayRef = Arc::new(Int32Array::from(vec![256; size]));
+
+        let array_args = vec![
+            ColumnarValue::Array(Arc::clone(&values)),
+            ColumnarValue::Array(Arc::clone(&bit_lengths)),
+        ];
+        run_benchmark(c, "sha2/array_binary_256", size, &array_args);
+
+        let array_scalar_args = vec![
+            ColumnarValue::Array(Arc::clone(&values)),
+            ColumnarValue::Scalar(ScalarValue::Int32(Some(256))),
+        ];
+        run_benchmark(c, "sha2/array_scalar_binary_256", size, &array_scalar_args);
+    }
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);

datafusion/spark/src/function/hash/sha2.rs

@@ -15,21 +15,22 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use arrow::array::{ArrayRef, AsArray, BinaryArrayType, Int32Array, StringArray};
+use arrow::array::{
+    ArrayRef, AsArray, BinaryArrayType, Int32Array, StringArray, new_null_array,
+};
 use arrow::datatypes::{DataType, Int32Type};
 use datafusion_common::types::{
     NativeType, logical_binary, logical_int32, logical_string,
 };
 use datafusion_common::utils::take_function_args;
-use datafusion_common::{Result, internal_err};
+use datafusion_common::{Result, ScalarValue, internal_err};
 use datafusion_expr::{
     Coercion, ColumnarValue, ScalarFunctionArgs, ScalarUDFImpl, Signature,
     TypeSignatureClass, Volatility,
 };
 use datafusion_functions::utils::make_scalar_function;
 use sha2::{self, Digest};
 use std::any::Any;
-use std::fmt::Write;
 use std::sync::Arc;
 
 /// Differs from DataFusion version in allowing array input for bit lengths, and
@@ -87,7 +88,97 @@ impl ScalarUDFImpl for SparkSha2 {
     }
 
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
-        make_scalar_function(sha2_impl, vec![])(&args.args)
+        let [values, bit_lengths] = take_function_args(self.name(), args.args.iter())?;
+
+        match (values, bit_lengths) {
+            (
+                ColumnarValue::Scalar(value_scalar),
+                ColumnarValue::Scalar(ScalarValue::Int32(Some(bit_length))),
+            ) => {
+                if value_scalar.is_null() {
+                    return Ok(ColumnarValue::Scalar(ScalarValue::Utf8(None)));
+                }
+
+                // Accept both Binary and Utf8 scalars (depending on coercion)
+                let bytes = match value_scalar {
+                    ScalarValue::Binary(Some(b)) => b.as_slice(),
+                    ScalarValue::LargeBinary(Some(b)) => b.as_slice(),
+                    ScalarValue::BinaryView(Some(b)) => b.as_slice(),
+                    ScalarValue::Utf8(Some(s))
+                    | ScalarValue::LargeUtf8(Some(s))
+                    | ScalarValue::Utf8View(Some(s)) => s.as_bytes(),
+                    other => {
+                        return internal_err!(
+                            "Unsupported scalar datatype for sha2: {}",
+                            other.data_type()
+                        );
+                    }
+                };
+
+                let out = match bit_length {
+                    224 => {
+                        let mut digest = sha2::Sha224::default();
+                        digest.update(bytes);
+                        Some(hex_encode(digest.finalize()))
+                    }
+                    0 | 256 => {
+                        let mut digest = sha2::Sha256::default();
+                        digest.update(bytes);
+                        Some(hex_encode(digest.finalize()))
+                    }
+                    384 => {
+                        let mut digest = sha2::Sha384::default();
+                        digest.update(bytes);
+                        Some(hex_encode(digest.finalize()))
+                    }
+                    512 => {
+                        let mut digest = sha2::Sha512::default();
+                        digest.update(bytes);
+                        Some(hex_encode(digest.finalize()))
+                    }
+                    _ => None,
+                };
+
+                Ok(ColumnarValue::Scalar(ScalarValue::Utf8(out)))
+            }
+            // Array values + scalar bit length (common case: sha2(col, 256))
+            (
+                ColumnarValue::Array(values_array),
+                ColumnarValue::Scalar(ScalarValue::Int32(Some(bit_length))),
+            ) => {
+                let output: ArrayRef = match values_array.data_type() {
+                    DataType::Binary => sha2_binary_scalar_bitlen(
+                        &values_array.as_binary::<i32>(),
+                        *bit_length,
+                    ),
+                    DataType::LargeBinary => sha2_binary_scalar_bitlen(
+                        &values_array.as_binary::<i64>(),
+                        *bit_length,
+                    ),
+                    DataType::BinaryView => sha2_binary_scalar_bitlen(
+                        &values_array.as_binary_view(),
+                        *bit_length,
+                    ),
+                    dt => return internal_err!("Unsupported datatype for sha2: {dt}"),
+                };
+                Ok(ColumnarValue::Array(output))
+            }
+            (
+                ColumnarValue::Scalar(_),
+                ColumnarValue::Scalar(ScalarValue::Int32(None)),
+            ) => Ok(ColumnarValue::Scalar(ScalarValue::Utf8(None))),
+            (
+                ColumnarValue::Array(_),
+                ColumnarValue::Scalar(ScalarValue::Int32(None)),
+            ) => Ok(ColumnarValue::Array(new_null_array(
+                &DataType::Utf8,
+                args.number_rows,
+            ))),
+            _ => {
+                // Fallback to existing behavior for any array/mixed cases
+                make_scalar_function(sha2_impl, vec![])(&args.args)
+            }
+        }
     }
 }
 
@@ -112,10 +203,31 @@ fn sha2_binary_impl<'a, BinaryArrType>(
 ) -> ArrayRef
 where
     BinaryArrType: BinaryArrayType<'a>,
+{
+    sha2_binary_bitlen_iter(values, bit_lengths.iter())
+}
+
+fn sha2_binary_scalar_bitlen<'a, BinaryArrType>(
+    values: &BinaryArrType,
+    bit_length: i32,
+) -> ArrayRef
+where
+    BinaryArrType: BinaryArrayType<'a>,
+{
+    sha2_binary_bitlen_iter(values, std::iter::repeat(Some(bit_length)))
+}
+
+fn sha2_binary_bitlen_iter<'a, BinaryArrType, I>(
+    values: &BinaryArrType,
+    bit_lengths: I,
+) -> ArrayRef
+where
+    BinaryArrType: BinaryArrayType<'a>,
+    I: Iterator<Item = Option<i32>>,
 {
     let array = values
         .iter()
-        .zip(bit_lengths.iter())
+        .zip(bit_lengths)
         .map(|(value, bit_length)| match (value, bit_length) {
             (Some(value), Some(224)) => {
                 let mut digest = sha2::Sha224::default();
@@ -144,11 +256,18 @@ where
     Arc::new(array)
 }
 
+const HEX_CHARS: [u8; 16] = *b"0123456789abcdef";
+
+#[inline]
 fn hex_encode<T: AsRef<[u8]>>(data: T) -> String {
-    let mut s = String::with_capacity(data.as_ref().len() * 2);
-    for b in data.as_ref() {
-        // Writing to a string never errors, so we can unwrap here.
-        write!(&mut s, "{b:02x}").unwrap();
+    let bytes = data.as_ref();
+    let mut out = Vec::with_capacity(bytes.len() * 2);
+    for &b in bytes {
+        let hi = b >> 4;
+        let lo = b & 0x0F;
+        out.push(HEX_CHARS[hi as usize]);
+        out.push(HEX_CHARS[lo as usize]);
     }
-    s
+    // SAFETY: out contains only ASCII
+    unsafe { String::from_utf8_unchecked(out) }
 }