Remek Zajac created ARROW-4542:
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Summary: Denominate row group size in bytes (not in no of rows)
Key: ARROW-4542
URL: https://issues.apache.org/jira/browse/ARROW-4542
Project: Apache Arrow
Issue Type: Improvement
Components: C++
Reporter: Remek Zajac
Both the C++ [implementation of parquet writer for
arrow|https://github.com/apache/arrow/blob/master/cpp/src/parquet/arrow/writer.cc#L1174]
and the [Python code bound to
it|https://github.com/apache/arrow/blob/master/python/pyarrow/_parquet.pyx#L911]
appears denominated in the *number of rows* (without making it very explicit).
Whereas:
(1) [The Apache parquet
documentation|https://parquet.apache.org/documentation/latest/] states:
"_Row group size: Larger row groups allow for larger column chunks which makes
it possible to do larger sequential IO. Larger groups also require more
buffering in the write path (or a two pass write). *We recommend large row
groups (512MB - 1GB)*. Since an entire row group might need to be read, we want
it to completely fit on one HDFS block. Therefore, HDFS block sizes should also
be set to be larger. An optimized read setup would be: 1GB row groups, 1GB HDFS
block size, 1 HDFS block per HDFS file._"
(2) Reference Apache [parquet-mr
implementation|https://github.com/apache/parquet-mr/blob/master/parquet-hadoop/src/main/java/org/apache/parquet/hadoop/InternalParquetRecordWriter.java#L146]
for Java accepts the row size expressed in bytes.
(3) The [low-level parquet read-write
example|https://github.com/apache/arrow/blob/master/cpp/examples/parquet/low-level-api/reader-writer2.cc#L88]
also considers row group be denominated in bytes.
These insights make me conclude that:
* Per parquet design and to take advantage of HDFS block level operations, it
only makes sense to work with row group sizes as expressed in bytes - as that
the only consequential desire the caller can express and want to influence.
* Arrow implementation of ParquetWriter would benefit from re-nominating its
`row_group_size` into bytes.
Now, my conclusions can be wrong and I may be blind to some alley of reasoning,
so this ticket is more of a question than a bug. A question on whether the
audience here agrees with my reasoning and if not - to explain what detail I
have missed.
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