[jira] [Created] (ARROW-4542) Denominate row group size in bytes (not in no of rows)

["Remek Zajac (JIRA)" <ji...@apache.org>]

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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