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https://issues.apache.org/jira/browse/CALCITE-1045?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17351156#comment-17351156
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Stamatis Zampetakis commented on CALCITE-1045:
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I've noticed that there is a PR#2385 by [~jamesstarr] linked to this issue. I
went quickly over the changes but I would appreciate some extra clarifications
regarding the high-level goal of this PR.
> Decorrelate sub-queries in Project and Join
> -------------------------------------------
>
> Key: CALCITE-1045
> URL: https://issues.apache.org/jira/browse/CALCITE-1045
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Priority: Major
> Labels: pull-request-available, sub-query
> Time Spent: 20m
> Remaining Estimate: 0h
>
> In CALCITE-816 we created a representation of sub-query in the relational
> algebra, RexSubQuery, so that we could defer expanding and de-correlating
> sub-queries until after SqlNode tree has been converted to RelNode tree. But
> the RelDecorrelator can only reliably handle sub-queries that occur in a
> Filter (such as WHERE EXISTS or WHERE IN).
> This task would except RelDecorrelator to handle sub-queries in Project
> (which occur with scalar sub-queries in the SELECT clause) and sub-queries in
> Join (which occur with scalar sub-queries in the ON clause of a JOIN).
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