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https://issues.apache.org/jira/browse/SPARK-32225?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17454622#comment-17454622
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Stijn De Haes commented on SPARK-32225:
---------------------------------------
Could this be the reason that when you read a Parquet data source and write it
back from S3 without doing anythong to the data then spark tells you it read
double the amount of data that it wrote:
!image-2021-12-07-13-37-12-197.png!
> Parquet footer information is read twice
> ----------------------------------------
>
> Key: SPARK-32225
> URL: https://issues.apache.org/jira/browse/SPARK-32225
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.0.0
> Reporter: Rajesh Balamohan
> Priority: Minor
> Attachments: image-2021-12-07-13-37-12-197.png,
> spark_parquet_footer_reads.png
>
>
> When running queries, spark reads parquet footer information twice. In cloud
> env, this would turn out to be expensive (depending on the jobs, # of
> splits). It would be nice to reuse the footer information already read via
> "ParquetInputFormat::buildReaderWithPartitionValues"
>
> !spark_parquet_footer_reads.png|width=640,height=644!
> Lines of interest:
> [https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala#L271]
> [https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala#L326]
>
> [https://github.com/apache/spark/blob/master/sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/SpecificParquetRecordReaderBase.java#L105]
> [https://github.com/apache/spark/blob/master/sql/core/src/main/java/org/apache/spark/sql/execution/datasources/parquet/SpecificParquetRecordReaderBase.java#L111]
>
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