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https://issues.apache.org/jira/browse/CALCITE-7608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18090992#comment-18090992
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Mihai Budiu commented on CALCITE-7608:
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I think I can explain this in a light that will make it more palatable:
consider a "lossless" aggregate function SELECT ARRAY_AGG(x) FROM T GROUP BY y.
The new proposed operator is the exact inverse of this form of aggregation,
allowing you to reconstitute the original table from the aggregated result.
This is something that cannot be done with just Uncollect. So this operator
really is the missing piece to make the algebra complete.
A version of flat_map would work too, but, as you pointed out, that is
significantly more powerful.
> 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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