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https://issues.apache.org/jira/browse/SPARK-25700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16646375#comment-16646375
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Apache Spark commented on SPARK-25700:
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User 'HyukjinKwon' has created a pull request for this issue:
https://github.com/apache/spark/pull/22697
> Avoid to create a readsupport at write path in Data Source V2
> -------------------------------------------------------------
>
> Key: SPARK-25700
> URL: https://issues.apache.org/jira/browse/SPARK-25700
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Affects Versions: 2.4.0
> Reporter: Hyukjin Kwon
> Priority: Critical
>
> https://github.com/apache/spark/commit/5fef6e3513d6023a837c427d183006d153c7102b
> introduced append mode support in Datasource V2.
> The problem is, write path ended up
> https://github.com/apache/spark/blob/e06da95cd9423f55cdb154a2778b0bddf7be984c/sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala#L249
> Retrieving the physical schema of the underlying storage is potentially
> expensive. Actually even worse: it looks odd that write path requires read
> side's schema.
> This is a regression comparing to Spark 2.3.
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