westonpace opened a new pull request #11616:
URL: https://github.com/apache/arrow/pull/11616
**This is still very much a WIP**
This PR attempts to address several issues:
* Memory mapped IPC reads always call WillNeed on the data and the user has
no way to avoid this
* Projection pushdown is only available in the synchronous API
* Coalescing / readahead is only available via the generators API
* There is a lot of duplicate code in the generators path
It adds two new methods to RecordBatchFileReader:
```
/// \brief Begin loading metadata for the desired batches into memory.
///
/// This method will also begin loading all dictionaries messages into
memory.
///
/// For a regular file this will immediately begin disk I/O in the
background on a
/// thread on the IOContext's thread pool. If the file is memory mapped
this will
/// ensure the memory needed for the metadata is paged from disk into
memory
///
/// \param indices Indices of the batches to prefetch
/// If empty then all batches will be prefetched.
virtual Status WillNeedMetadata(const std::vector<int>& indices) = 0;
/// \brief Begin loading metadata for the desired batches into memory and
indicate
/// that the data itself should be prefetched when it is requested
///
/// This method should not be called in combination with WillNeedMetadata.
If you want
/// to prefetch the data then use this method. If you do not want to
prefetch the data
/// (because you are only accessing a small # of items in the batch's
arrays) then you
/// should use WillNeedMetadata
///
/// This method will immediately start the I/O for the metadata and
dictionaries.
///
/// This method will not immediately start the I/O for the data. The data
I/O will be
/// started when you call ReadRecordBatch.
///
/// If you want to read multiple batches in parallel then you can make
concurrent calls
/// to ReadRecordBatch or ReadRecordBatchAsync
/// \param indices
/// \return
virtual Status WillNeedBatches(const std::vector<int>& indices) = 0;
```
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