westonpace commented on code in PR #36779:
URL: https://github.com/apache/arrow/pull/36779#discussion_r1296156006
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cpp/src/parquet/arrow/reader.h:
##########
@@ -249,6 +249,90 @@ class PARQUET_EXPORT FileReader {
virtual ::arrow::Status ReadRowGroups(const std::vector<int>& row_groups,
std::shared_ptr<::arrow::Table>* out)
= 0;
+ using AsyncBatchGenerator =
+ std::function<::arrow::Future<std::shared_ptr<::arrow::RecordBatch>>()>;
+
+ /// \brief Read a single row group from the file
+ ///
+ /// \see ReadRowGroupsAsync for operation details
+ ///
+ /// \param i the index of the row group to read
+ /// \param cpu_executor an executor to use to run CPU tasks
+ /// \param allow_sliced_batches if false, an error is raised if a batch has
too much
+ /// data for the given batch size. If true,
smaller
+ /// batches will be returned instead.
+ virtual AsyncBatchGenerator ReadRowGroupAsync(int i,
+ ::arrow::internal::Executor*
cpu_executor,
+ bool allow_sliced_batches =
false) = 0;
+ /// \brief Read some columns from a single row group from the file
+ ///
+ /// \see ReadRowGroupsAsync for operation details
+ /// \see ReadTable for details on how column indices are resolved
+ ///
+ /// \param i the index of the row group to read
+ /// \param column_indices leaf-indices of the columns to read
+ /// \param cpu_executor an executor to use to run CPU tasks
+ /// \param allow_sliced_batches if false, an error is raised if a batch has
too much
+ /// data for the given batch size. If true,
smaller
+ /// batches will be returned instead.
+ virtual AsyncBatchGenerator ReadRowGroupAsync(int i,
+ const std::vector<int>&
column_indices,
+ ::arrow::internal::Executor*
cpu_executor,
+ bool allow_sliced_batches =
false) = 0;
+
+ /// \brief Read row groups from the file
+ ///
+ /// \see ReadRowGroupsAsync for operation details
+ ///
+ /// \param row_groups indices of the row groups to read
+ /// \param cpu_executor an executor to use to run CPU tasks
+ /// \param allow_sliced_batches if false, an error is raised if a batch has
too much
+ /// data for the given batch size. If true,
smaller
+ /// batches will be returned instead.
+ virtual AsyncBatchGenerator ReadRowGroupsAsync(
+ const std::vector<int>& row_groups, ::arrow::internal::Executor*
cpu_executor,
+ bool allow_sliced_batches = false) = 0;
+
+ /// \brief Read some columns from the given rows groups from the file
+ ///
+ /// If pre-buffering is enabled then all of the data will be read using the
pre-buffer
+ /// cache. See ParquetFileReader::PreBuffer for details on how this affects
memory and
+ /// performance.
+ ///
+ /// This operation is not perfectly async. The read from disk will be done
on an I/O
+ /// thread, which is correct. However, compression and column decoding is
also done on
+ /// the I/O thread which may not be ideal. The stage after that
(transferring the
+ /// decoded data into Arrow structures and fulfilling the future) should be
done as a
+ /// new task on the cpu_executor.
+ ///
+ /// The returned generator will respect the batch size set in the
ArrowReaderProperties.
+ /// Batches will not be larger than the given batch size. However, batches
may be
+ /// smaller. This can happen, for example, when there is not enough data or
when a
+ /// string column is too large to fit into a single batch. The parameter
+ /// `allow_sliced_batches` can be set to false to disallow this later case.
This can be
+ /// useful when you need to know exactly how many batches you will get from
the
+ /// operation before you start.
+ ///
+ /// Note: When reading multiple row groups there is no guarantee you will
get one
+ /// record batch per row group. Data from multiple row groups could get
combined into
+ /// a single batch.
Review Comment:
No. This is the same way that the synchronous APIs behave.
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