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GH-55305: [C++][Parquet] Avoid unnecessary copies when building data pages #55306

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Merged
HuaHuaY merged 2 commits into from
Nov 2, 2026
Merged

GH-55305: [C++][Parquet] Avoid unnecessary copies when building data pages #55306

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f6bcd5e
avoid unnecessary copies when building data pages
HuaHuaY Oct 29, 2026
da8eb30
address review comments
HuaHuaY Oct 29, 2026
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20 changes: 11 additions & 9 deletions cpp/src/parquet/column_writer.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1004,22 +1004,26 @@ void ColumnWriterImpl::BuildDataPageV1(int64_t definition_levels_rle_size,
int64_t repetition_levels_rle_size,
int64_t uncompressed_size,
const std::shared_ptr<Buffer>& values) {
// Use Arrow::Buffer::shrink_to_fit = false
// underlying buffer only keeps growing. Resize to a smaller size does not reallocate.
PARQUET_THROW_NOT_OK(uncompressed_data_->Resize(uncompressed_size, false));
ConcatenateBuffers(definition_levels_rle_size, repetition_levels_rle_size, values,
uncompressed_data_->mutable_data());
auto uncompressed_data = values;
if (definition_levels_rle_size > 0 || repetition_levels_rle_size > 0) {
// Underlying buffer only keeps growing. Resize to a smaller size does not reallocate.
PARQUET_THROW_NOT_OK(
uncompressed_data_->Resize(uncompressed_size, /*shrink_to_fit=*/false));
ConcatenateBuffers(definition_levels_rle_size, repetition_levels_rle_size, values,
uncompressed_data_->mutable_data());
uncompressed_data = uncompressed_data_;
}
auto [page_stats, page_size_stats] = GetPageStatistics();
page_stats.ApplyStatSizeLimits(properties_->max_statistics_size(descr_->path()));
page_stats.set_is_signed(SortOrder::SIGNED == descr_->sort_order());
ResetPageStatistics();

std::shared_ptr<Buffer> compressed_data;
if (pager_->has_compressor()) {
pager_->Compress(*(uncompressed_data_.get()), compressor_temp_buffer_.get());
pager_->Compress(*uncompressed_data, compressor_temp_buffer_.get());
compressed_data = compressor_temp_buffer_;
} else {
compressed_data = uncompressed_data_;
compressed_data = std::move(uncompressed_data);
}

int32_t num_values = static_cast<int32_t>(num_buffered_values_);
Expand Down Expand Up @@ -1089,8 +1093,6 @@ void ColumnWriterImpl::BuildDataPageV2(int64_t definition_levels_rle_size,
// Write the page to OutputStream eagerly if there is no dictionary or
// if dictionary encoding has fallen back to PLAIN
if (has_dictionary_ && !fallback_) { // Save pages until end of dictionary encoding
PARQUET_ASSIGN_OR_THROW(auto data_copy,
combined->CopySlice(0, combined->size(), allocator_));

@pitrou pitrou Oct 29, 2026

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So data_copy was unused? 🤨

@HuaHuaY HuaHuaY Oct 29, 2026 •
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I checked the git history and suspect that the author of BuildDataPageV2 copied BuildDataPageV1 and modified it, failing to notice the unused variable. Meanwhile, clang-tidy didn't flag any warning here either.

std::unique_ptr<DataPage> page_ptr = std::make_unique<DataPageV2>(
combined, num_values, null_count, num_rows, encoding_, def_levels_byte_length,
rep_levels_byte_length, uncompressed_size, page_is_compressed,
Expand Down
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