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https://issues.apache.org/jira/browse/CALCITE-7608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18099020#comment-18099020
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Mihai Budiu commented on CALCITE-7608:
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I see that no one wants to review this, but I would like to make progress.
I will break this issue (and the PR) into 3 separate issues, two of which are
really bug fixes for the current Uncollect operator. Maybe this will make the
final PR (Enhance Uncollect) more palatable.
> Enhance Uncollect
> ------------------
>
> Key: CALCITE-7608
> URL: https://issues.apache.org/jira/browse/CALCITE-7608
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.42.0
> Reporter: Mihai Budiu
> Assignee: Mihai Budiu
> Priority: Minor
> Labels: pull-request-available
>
> Today SQL UNNEST is implemented using the Uncollect operator. We propose
> adding two additional capabilities to Uncollect, which generalize its current
> behavior:
> * support for LEFT JOIN UNNEST
> * support for carrying over non-collection input fields to the output
> unchanged
> The first capability is needed because the semantics of UNCOLLECT on the
> right-hand-side of a LEFT JOIN cannot be implemented using the existing
> semantics: UNCOLLECT will need to produce a NULL for empty or NULL
> collections in such cases.
> The second capability is independent on the first one: it would strictly
> increase the expressive power of Uncollect, allowing it to not only Unnest
> collections, but also copy some input fields unchanged for every element of
> an unnested collection.
> The main reason for the second capability is that it would enable us to
> represent plans using SQL UNNEST without using Correlate nodes. A sub-plan
> containing Project + Correlate + Uncollect + Values would become a single
> Uncollect node. Neither the old nor the new decorrelator can actually
> eliminate Correlate + Uncollect (there is no other representation to express
> the same behavior). Using the new representation we can decorrelate many more
> plans (perhaps all plans).
> Although these two changes are logically independent, I propose to make them
> in a single combined PR, because they would both change the type checking and
> runtime implementation of the operator, and it is desirable to minimize the
> churn on consumers of the new operator.
> A temporary configuration flag would be introduced to control whether
> SqlToRelConverter uses the new form of the operator, or just the old form.
> Hopefully programs that only use the old form would remain unchanged. This
> will give Calcite users time to upgrade to the new representation at their
> own pace.
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