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https://issues.apache.org/jira/browse/CALCITE-7608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18090444#comment-18090444
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Mihai Budiu commented on CALCITE-7608:
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I actually think Uncollect is the wrong one, "SelectMany" or "ProjectMany" is
the right abstraction. Uncollect should be deprecated. Think of this new
operator as replacing Uncollect.
The only rewrite rules that will be affected are really the ones that were
looking for Uncollect, and there are very few of these.
The Decorrelator is one of the most complicated pieces of Calcite; moreover, if
the goal is for the decorrelator to produce a plan without any Correlate calls,
it's impossible if you have only Uncollect as a primitive.
> Introduce a SelectMany operator
> -------------------------------
>
> 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 UNNEST is implemented using the Uncollect operator. We propose adding
> an alternative LogicalSelectMany operator, which generalizes Uncollect.
> (Notice that Enumerable API already has a SelectMany.) The main difference
> between Uncollect and SelectMany is that Uncollect unnests all the fields of
> its input relation, whereas LogicalSelectMany would only unnest SOME of the
> fields of the input collection, preserving the other ones in each output row.
> This distinction is very important, because:
> * LogicalSelectMany can be directly and efficiently implemented using the
> Enumerable SelectMany
> * UNNEST used in a cross-join is implemented using an Uncollect and a
> LogicalCorrelate. However, the same UNNEST can be represented using just one
> LogicalSelectMany node
> * Neither the old nor the new decorrelator can actually eliminate
> LogicalCorrelate nodes that are paired with Uncollect. Using
> LogicalSelectMany we can decorrelate many more plans.
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