Copilot commented on code in PR #51362:
URL: https://github.com/apache/arrow/pull/51362#discussion_r4029794348
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cpp/src/parquet/arrow/reader.cc:
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@@ -1379,11 +1379,18 @@ Future<std::shared_ptr<Table>>
FileReaderImpl::DecodeRowGroups(
// OptionalParallelForAsync requires an executor
if (!cpu_executor) cpu_executor = ::arrow::internal::GetCpuThreadPool();
- auto read_column = [row_groups, self, this](size_t i,
-
std::shared_ptr<ColumnReaderImpl> reader)
+ // `readers` only holds the requested columns, so its index `i` is a
position within
+ // the selection, not the column's actual index in the row group. Map back
to the
+ // real column index (as GetFieldReaders does internally) before calling
ReadColumn,
+ // which indexes RowGroupMetaData::ColumnChunk() by the latter.
+ ARROW_ASSIGN_OR_RAISE(std::vector<int> field_indices,
+ manifest_.GetFieldIndices(column_indices));
Review Comment:
`GetFieldIndices` returns top-level/root field positions, not Parquet
leaf-column indices (see `schema.h:139-144`), while `ReadColumn` uses its
argument in `RowGroup::ColumnChunk(i)` and the public API defines requested
indices as leaves (`reader.h:202-218`). For a schema such as `struct a {x, y},
c`, selecting `c` yields `column_indices=[2]` and `field_indices=[1]`, so this
still reads `ColumnChunk(1)` (`a.y`), potentially decrypting the wrong column
or using its value count. Build one selected leaf index per root reader and add
a nested-schema regression instead of passing `field_indices` here.
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