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https://issues.apache.org/jira/browse/HIVE-25827?focusedWorklogId=781116&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-781116
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ASF GitHub Bot logged work on HIVE-25827:
-----------------------------------------
Author: ASF GitHub Bot
Created on: 14/Jun/22 11:46
Start Date: 14/Jun/22 11:46
Worklog Time Spent: 10m
Work Description: szlta opened a new pull request, #3368:
URL: https://github.com/apache/hive/pull/3368
This change refactors the Parquet record reader implementations in Hive
(ParquetRecordReaderWrapper, VectorizedParquetRecordReader and their base
class: ParquetRecordReaderBase). The way it's currently done is that the file
footer is read at least 2 times.
Also we don't need multiple splits of the same file for this to happen.
Every file is opened 2 times with the vectorized reader. And yes, e.g. in
LLAP's case that's two different streams as in that scenario there's a custom
logic to load the metadata part of the file into metadata cache. If there's a
cache hit, 1 of the reads would be served from cache, but the other occasion
reads from file in the dumb way nevertheless.
Key changes:
- After the refactor the Parquet metadata loading happens via a virtual
method instead and the result is stored in a field so depending on whether the
job is non-vectorized/vectorized/llap this now can be done in separate ways.
- Also did some cleanup and restructured common logic into
ParquetRecordReaderBase.
- Originally VectorizedParquetRecordReader had a code path for when `if
(rowGroupOffsets == null) {`. I don't think this is a valid scenario and even
if it was it wouldn't have worked correctly ever: in `range(split.getStart(),
split.getEnd())` the split instance is of a ParquetInputSplit, where `getEnd()`
calculates how many bytes would be read in that split (summed), and it doesn't
denote a position. Therefore, projections where some columns are omitted would
produce a bad result. So.., this code path is removed in this refactor too.
Issue Time Tracking
-------------------
Worklog Id: (was: 781116)
Remaining Estimate: 0h
Time Spent: 10m
> Parquet file footer is read multiple times, when multiple splits are created
> in same file
> -----------------------------------------------------------------------------------------
>
> Key: HIVE-25827
> URL: https://issues.apache.org/jira/browse/HIVE-25827
> Project: Hive
> Issue Type: Improvement
> Reporter: Rajesh Balamohan
> Assignee: Ádám Szita
> Priority: Major
> Labels: performance
> Attachments: image-2021-12-21-03-19-38-577.png
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> With large files, it is possible that multiple splits are created in the same
> file. With current codebase, "ParquetRecordReaderBase" ends up reading file
> footer for each split.
> It can be optimized not to read footer information multiple times for the
> same file.
>
> [https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/io/parquet/vector/VectorizedParquetRecordReader.java#L160]
>
> [https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/io/parquet/ParquetRecordReaderBase.java#L91]
>
>
> !image-2021-12-21-03-19-38-577.png|width=1363,height=1256!
>
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