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https://issues.apache.org/jira/browse/HIVE-29380?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18046351#comment-18046351
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Stamatis Zampetakis commented on HIVE-29380:
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The presence of specialized "synthetic" projections complicates potential
improvements on common table expression detection. I discovered this issue
while working on generating smarter suggestions for TPC-DS Q9 in HIVE-29374.
> Drop synthetic HiveProject and HiveFilterProjectTSTransposeRule
> ---------------------------------------------------------------
>
> Key: HIVE-29380
> URL: https://issues.apache.org/jira/browse/HIVE-29380
> Project: Hive
> Issue Type: Task
> Reporter: Stamatis Zampetakis
> Assignee: Stamatis Zampetakis
> Priority: Major
>
> The [synthetic indicator in
> HiveProject|https://github.com/apache/hive/blob/ee7138bf7a1a1ee1de07fa2243fe947a627268cb/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/reloperators/HiveProject.java#L49]
> was introduced alongside HiveFilterProjectTSTransposeRule in HIVE-11110 as
> way to restrict the application of project pushdown (PPD) to a limited number
> of cases. The rule was made to apply only on projections directly over a
> table scan that have been introduced after field trimming.
> In fact, the synthetic indicator is completely redundant since if we restrict
> the rule matches strictly to projections over table scans we can have the
> same outcome without the need to maintain an explicit indicator that
> specializes the behavior of the projection operator.
> Dropping the redundant logic and rule reduces code maintenance and tech-debt
> and facilitates the detection of equivalent expressions/plans since there are
> no multiple versions of a project operator.
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