mapleFU commented on code in PR #36774:
URL: https://github.com/apache/arrow/pull/36774#discussion_r1268338192
##########
cpp/src/parquet/file_reader.cc:
##########
@@ -366,13 +366,19 @@ class SerializedFile : public ParquetFileReader::Contents
{
std::make_shared<::arrow::io::internal::ReadRangeCache>(source_, ctx,
options);
std::vector<::arrow::io::ReadRange> ranges;
prebuffered_column_chunks_.clear();
+ int num_cols = file_metadata_->num_columns();
+ // a bitmap for buffered columns.
+ std::shared_ptr<Buffer> buffer_columns;
+ if (!row_groups.empty()) {
Review Comment:
a possible optimization is that we can regard `nullptr` as buffer the column.
```c++
if (!row_groups.empty()) {
if (num_cols == static_cast<int>(row_groups.size())) {
buffer_columns = nullptr;
} else {
PARQUET_THROW_NOT_OK(AllocateEmptyBitmap(num_cols,
properties_.memory_pool())
.Value(&buffer_columns));
for (int col : column_indices) {
::arrow::bit_util::SetBit(buffer_columns->mutable_data(), col);
}
}
}
```
And if
```
void PreBuffer(const std::vector<int>& row_groups,
const std::vector<int>& column_indices,
const ::arrow::io::IOContext& ctx,
const ::arrow::io::CacheOptions& options)
```
Find that the `buffer` is nullptr, it will just buffer the column.
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