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ARROW-10403: [C++] Implement unique kernel for non-uniform chunked dictionary arrays #9683

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ARROW-10403: [C++] Implement unique kernel for non-uniform chunked dictionary arrays #9683

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ec9dd24
Changing DictionaryHashKernel.
rok Mar 12, 2021
eb63876
Add comment about complexity
pitrou Apr 8, 2021
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37 changes: 27 additions & 10 deletions cpp/src/arrow/compute/kernels/vector_hash.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@
#include "arrow/array/array_dict.h"
#include "arrow/array/array_nested.h"
#include "arrow/array/builder_primitive.h"
#include "arrow/array/concatenate.h"
#include "arrow/array/dict_internal.h"
#include "arrow/array/util.h"
#include "arrow/compute/api_vector.h"
Expand Down Expand Up @@ -440,19 +441,35 @@ class DictionaryHashKernel : public HashKernel {

Status Reset() override { return indices_kernel_->Reset(); }

Status HandleDictionary(const std::shared_ptr<ArrayData>& dict) {
Status Append(const ArrayData& arr) override {
if (!dictionary_) {
dictionary_ = dict;
} else if (!MakeArray(dictionary_)->Equals(*MakeArray(dict))) {
return Status::Invalid(
"Only hashing for data with equal dictionaries "
"currently supported");
dictionary_ = arr.dictionary;
} else if (!MakeArray(dictionary_)->Equals(*MakeArray(arr.dictionary))) {
// NOTE: This approach computes a new dictionary unification per chunk.
// This is in effect O(n*k) where n is the total chunked array length and
// k is the number of chunks (therefore O(n**2) if chunks have a fixed size).
//
// A better approach may be to run the kernel over each individual chunk,
// and then hash-aggregate all results (for example sum-group-by for
// the "value_counts" kernel).
auto out_dict_type = dictionary_->type;
std::shared_ptr<Buffer> transpose_map;
std::shared_ptr<Array> out_dict;
ARROW_ASSIGN_OR_RAISE(auto unifier, DictionaryUnifier::Make(out_dict_type));

ARROW_CHECK_OK(unifier->Unify(*MakeArray(dictionary_)));
ARROW_CHECK_OK(unifier->Unify(*MakeArray(arr.dictionary), &transpose_map));
ARROW_CHECK_OK(unifier->GetResult(&out_dict_type, &out_dict));

this->dictionary_ = out_dict->data();
auto transpose = reinterpret_cast<const int32_t*>(transpose_map->data());
auto in_dict_array = MakeArray(std::make_shared<ArrayData>(arr));
ARROW_ASSIGN_OR_RAISE(
auto tmp, arrow::internal::checked_cast<const DictionaryArray&>(*in_dict_array)
.Transpose(arr.type, out_dict, transpose));
return indices_kernel_->Append(*tmp->data());
}
return Status::OK();
}

Status Append(const ArrayData& arr) override {
RETURN_NOT_OK(HandleDictionary(arr.dictionary));
return indices_kernel_->Append(arr);
}

Expand Down
15 changes: 9 additions & 6 deletions cpp/src/arrow/compute/kernels/vector_hash_test.cc
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Expand Up @@ -591,12 +591,15 @@ TEST_F(TestHashKernel, DictionaryUniqueAndValueCounts) {
CheckUnique(chunked, ex_uniques);
CheckValueCounts(chunked, ex_uniques, ex_counts);

// Different dictionaries not supported
auto dict2 = ArrayFromJSON(int64(), "[30, 40, 50, 60]");
auto input2 = std::make_shared<DictionaryArray>(dict_ty, indices, dict2);
auto different_dictionaries = *ChunkedArray::Make({input, input2});
ASSERT_RAISES(Invalid, Unique(different_dictionaries));
ASSERT_RAISES(Invalid, ValueCounts(different_dictionaries));
// Different chunk dictionaries
auto input_2 = DictArrayFromJSON(dict_ty, "[1, null, 2, 3]", "[30, 40, 50, 60]");
auto ex_uniques_2 =
DictArrayFromJSON(dict_ty, "[3, 0, 1, null, 4, 5]", "[10, 20, 30, 40, 50, 60]");
auto ex_counts_2 = ArrayFromJSON(int64(), "[4, 5, 4, 1, 1, 1]");
auto different_dictionaries = *ChunkedArray::Make({input, input_2}, dict_ty);

CheckUnique(different_dictionaries, ex_uniques_2);
CheckValueCounts(different_dictionaries, ex_uniques_2, ex_counts_2);

// Dictionary with encoded nulls
auto dict_with_null = ArrayFromJSON(int64(), "[10, null, 30, 40]");
Expand Down