[jira] [Created] (ARROW-4503) [C#] ArrowStreamReader allocates and copies data excessively

["Eric Erhardt (JIRA)" <ji...@apache.org>]

Eric Erhardt created ARROW-4503:
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             Summary: [C#] ArrowStreamReader allocates and copies data excessively
                 Key: ARROW-4503
                 URL: https://issues.apache.org/jira/browse/ARROW-4503
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C#
            Reporter: Eric Erhardt


When reading `RecordBatch` instances using the `ArrowStreamReader` class, it is currently allocating and copying memory 3 times for the data.
 # It is allocating memory in order to [read the data from the Stream|https://github.com/apache/arrow/blob/044b418fa108a57f0b4e2e887546cc3e68271397/csharp/src/Apache.Arrow/Ipc/ArrowStreamReader.cs#L72-L74], and then reading from the Stream.  (This should be the only allocation that is necessary.)
 # It then [creates a new `ArrowBuffer.Builder`|https://github.com/apache/arrow/blob/044b418fa108a57f0b4e2e887546cc3e68271397/csharp/src/Apache.Arrow/Ipc/ArrowStreamReader.cs#L227-L228], which allocates another `byte[]`, and calls `Append` on it, which copies the values to the new `byte[]`.
 # Finally, it then calls `.Build()` on the `ArrowBuffer.Builder`, which [allocates memory from the MemoryPool, and then copies the intermediate buffer|https://github.com/apache/arrow/blob/044b418fa108a57f0b4e2e887546cc3e68271397/csharp/src/Apache.Arrow/ArrowBuffer.Builder.cs#L112-L121] into it.

 

We should reduce this overhead to only allocating a single time (from the MemoryPool), and not copying the data more times than necessary.



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