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https://issues.apache.org/jira/browse/ARROW-4542?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16766188#comment-16766188
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Remek Zajac commented on ARROW-4542:
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Thanks for the prompt feedback [~wesmckinn] - glad you agree with my reasoning.
Sure - i know it ain't trivial. I saw what parquet-mr does. I will wait for the
ticket to cool off a bit and/or collect more feedback. And thanks for inviting
my contribution. We may find time to help here depending how much it ends up
hurting us compared to other places that hurt too.
> [C++] 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
> Priority: Major
>
> 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 is the only consequential desire the caller can utter and want to
> influence.
> * Arrow implementation of ParquetWriter would benefit from re-nominating its
> `row_group_size` into bytes. I will also note it is impossible to use pyarrow
> to shape equally byte-sized row groups as the size the row group takes is
> post-compression and the caller only know how much uncompressed data they
> have managed to put in.
> 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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