perf: skip validation of dictionary keys if all null (#9322)

# Which issue does this PR close?

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- Closes #9321 .

# 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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Improve the performance of creating a dictionary array that is all
nulls.

# What changes are included in this PR?

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is sometimes worth providing a summary of the individual changes in this
PR.
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The `try_new` constructor skips the key validation if all the keys are
null.

Adds a benchmark for creating the all null dictionary to verify the
performance improvement.

| Benchmark | Before | After | Change |
|-----------|--------|-------|---------|
| null_dict/len=128 | 133.37 ns | 98.222 ns | -25.659% |
| null_dict/len=1536 | 623.39 ns | 286.11 ns | -57.361% |
| null_dict/len=8092 | 3.0252 µs | 1.0719 µs | -66.573% |

# Are these changes tested?

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I added a smoke test that calls the constructor with an all nulls key
column and asserts the result is OK.

I wasn't sure about the best way to test this (open to suggestions).

# Are there any user-facing changes?

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no

Author: albertlockett

arrow-array/Cargo.toml

@@ -75,6 +75,10 @@ harness = false
 name = "fixed_size_list_array"
 harness = false
 
+[[bench]]
+name = "null_dict"
+harness = false
+
 [[bench]]
 name = "decimal_overflow"
 harness = false
@@ -85,4 +89,4 @@ harness = false
 
 [[bench]]
 name = "record_batch"
-harness = false
+harness = false

arrow-array/benches/null_dict.rs

@@ -0,0 +1,41 @@
+// 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.
+use arrow_array::{DictionaryArray, StringArray, UInt16Array};
+use std::sync::Arc;
+
+use criterion::*;
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let dict_vals = Arc::new(StringArray::from_iter_values(["a", "b", "c"]));
+    for len in [128, 1536, 8092] {
+        c.bench_function(&format!("null_dict/len={len}"), |b| {
+            b.iter_batched(
+                || dict_vals.clone(),
+                |dict_vals| {
+                    std::hint::black_box(DictionaryArray::new(
+                        UInt16Array::new_null(len),
+                        dict_vals.clone(),
+                    ))
+                },
+                BatchSize::SmallInput,
+            );
+        });
+    }
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);

arrow-array/src/array/dictionary_array.rs

@@ -292,17 +292,20 @@ impl<K: ArrowDictionaryKeyType> DictionaryArray<K> {
             Box::new(values.data_type().clone()),
         );
 
-        let zero = K::Native::usize_as(0);
-        let values_len = values.len();
+        // // we can skip the validating the keys if they are all null
+        let all_null = keys.null_count() == keys.len();
 
-        if let Some((idx, v)) =
-            keys.values().iter().enumerate().find(|(idx, v)| {
+        if !all_null {
+            let zero = K::Native::usize_as(0);
+            let values_len = values.len();
+
+            if let Some((idx, v)) = keys.values().iter().enumerate().find(|(idx, v)| {
                 (v.is_lt(zero) || v.as_usize() >= values_len) && keys.is_valid(*idx)
-            })
-        {
-            return Err(ArrowError::InvalidArgumentError(format!(
-                "Invalid dictionary key {v:?} at index {idx}, expected 0 <= key < {values_len}",
-            )));
+            }) {
+                return Err(ArrowError::InvalidArgumentError(format!(
+                    "Invalid dictionary key {v:?} at index {idx}, expected 0 <= key < {values_len}",
+                )));
+            }
         }
 
         Ok(Self {
@@ -1047,7 +1050,7 @@ impl<K: ArrowDictionaryKeyType> AnyDictionaryArray for DictionaryArray<K> {
 mod tests {
     use super::*;
     use crate::cast::as_dictionary_array;
-    use crate::{Int8Array, Int16Array, Int32Array, RunArray};
+    use crate::{Int8Array, Int16Array, Int32Array, RunArray, UInt8Array};
     use arrow_buffer::{Buffer, ToByteSlice};
 
     #[test]
@@ -1528,4 +1531,13 @@ mod tests {
         let dictionary = DictionaryArray::new(keys, Arc::new(Int32Array::new_null(2)));
         assert_eq!(&dictionary.normalized_keys(), &[1, 0, 1])
     }
+
+    #[test]
+    fn test_all_null_dict() {
+        let all_null_dict_arr = DictionaryArray::try_new(
+            UInt8Array::new_null(10),
+            Arc::new(StringArray::from_iter_values(["a"])),
+        );
+        assert!(all_null_dict_arr.is_ok())
+    }
 }