Getting record batch size with pa.get_record_batch_size returns a size that is too small for pandas DataFrame.

[asfimport]

I'm running into the following problem.

Suppose I create a dataframe and turn it into a record batch.

import pyarrow as pa
import pandas as pd

df = pd.DataFrame({"a": [1, 2, 3]})
record_batch = pa.RecordBatch.from_pandas(df)

It's size is 352 according to

pa.get_record_batch_size(record_batch)  # This is 352.

However, if I write it using a stream_writer and then attempt to read it, the resulting buffer has size 928.

sink = pa.BufferOutputStream()
stream_writer = pa.RecordBatchStreamWriter(sink, record_batch.schema)
stream_writer.write_batch(record_batch)
new_buf = sink.get_result()
new_buf.size  # This is 928.

I'm running into this problem because I'm attempting to write the pandas DataFrame to the Plasma object store as follows (after Plasma has been started and a client has been created), so I need to know the size ahead of time.

data_size = pa.get_record_batch_size(record_batch)
object_id = plasma.ObjectID(np.random.bytes(20))

buf = client.create(object_id, data_size)  # Note that if I replace "data_size" by "data_size + 1000" then it works.
stream = plasma.FixedSizeBufferOutputStream(buf)
stream_writer = pa.RecordBatchStreamWriter(stream, record_batch.schema)
stream_writer.write_batch(record_batch)

The above fails because the buffer allocated in Plasma only has size 352, but 928 bytes are needed.

So my question is, am I determining the size of the record batch incorrectly? Or could there be a bug in pa.get_record_batch_size?

Environment: Ubuntu 16.04
Python 3.6
Reporter: Robert Nishihara / @robertnishihara

Related issues:

• Arrow 0.5.0 release (blocks)

_{Note: This issue was originally created as ARROW-1194. Please see the migration documentation for further details.}

[asfimport]

Robert Nishihara / @robertnishihara:
Also, does 928 seem too big? Or does that sound about right?

[asfimport]

Wes McKinney / @wesm:
That does seem a little odd. The size would be larger than 352 to account for the schema, but 928 seems too large. So there might be a bug in the record batch size calculation, but separately we should add some size calculation functions that account for the total size of the stream payload including the schema.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 830
#830