[ARROW-1194] Getting record batch size with pa.get_record_batch_size returns a size that is too small for pandas DataFrame.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.5.0
• Fix Version/s: 0.5.0
• Component/s: Python
• Environment: Ubuntu 16.04; Python 3.6
• External issue URL: https://github.com/apache/arrow/issues/15432
• Assignee: Unassigned
• Reporter: Robert Nishihara
• Created: 07/Jul/17 19:39
• Updated: 11/Jan/23 07:13
• Resolved: 13/Jul/17 12:26

Description

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?

Issue Links

• blocks: ARROW-1052 Arrow 0.5.0 release