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https://issues.apache.org/jira/browse/SPARK-36770?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Apache Spark reassigned SPARK-36770:
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Assignee: Apache Spark
> Improve run-time performance for window function first and last against
> UnboundedFollowing window ROWS frame
> ------------------------------------------------------------------------------------------------------------
>
> Key: SPARK-36770
> URL: https://issues.apache.org/jira/browse/SPARK-36770
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.4.0, 3.1.2
> Reporter: Guibin Zhang
> Assignee: Apache Spark
> Priority: Major
>
> *Context*
> The *UnboundedFollowingWindowFunctionFrame* has the time complexity of
> *O(N^2)*, N is the number of rows in the current partition, more specific the
> complexity is O(N* (N - 1)/2).
> What happens internally in UnboundedFollowingWindowFunctionFrame:
> In the window frame, while processing each incoming row, it will go through
> current row till the end of partition to do re-calculation. This process will
> be repeated on each incoming row, which causes the high run-time complexity.
> But UnboundedPrecedingWindowFunctionFrame has much better time complexity
> O(N), N is the number of rows in the current partition.
>
> *What is the idea of the improvement?*
> Give the big time complexity difference between
> UnboundedFollowingWindowFunctionFrame and
> UnboundedPrecedingWindowFunctionFrame, we can do following conversions to
> improve the time complexity of first() and last() from O(N^2) to O(N)
> {code:java}
> case 1:
> first() OVER(PARTITION BY colA ORDER BY colB ASC ROWS UNBOUNDED FOLLOWING)
> converts to
> last() OVER(PARTITION BY colA ORDER BY colB DEAC ROWS UNBOUNDED PRECEDING)
> case 2:
> last() OVER(PARTITION BY colA ORDER BY colB ASC ROWS UNBOUNDED FOLLOWING)
> converts to
> first() OVER(PARTITION BY colA ORDER BY colB DESC ROWS UNBOUNDED PRECEDING)
> {code}
>
> *Summary*
> Replace "UNBOUNDED FOLLOWING" with "UNBOUNDED PRECEDING", and flip the ORDER
> BY for the window functions first() and last() for ROWS.
>
>
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