[jira] [Commented] (CALCITE-7608) Introduce a SelectMany operator

["Mihai Budiu (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-7608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18089779#comment-18089779 ] 

Mihai Budiu commented on CALCITE-7608:
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We will first need to merge CALCITE-7583 because it will contains some useful pieces and we need to check that the semantics of UNNEST does not change with this implementation.

> 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
>
> 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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