[jira] [Resolved] (ARROW-11174) [C++][Dataset] Make Expressions available for projection

["Neal Richardson (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-11174?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Neal Richardson resolved ARROW-11174.
-------------------------------------
    Resolution: Fixed

Issue resolved by pull request 9532
[https://github.com/apache/arrow/pull/9532]

> [C++][Dataset] Make Expressions available for projection
> --------------------------------------------------------
>
>                 Key: ARROW-11174
>                 URL: https://issues.apache.org/jira/browse/ARROW-11174
>             Project: Apache Arrow
>          Issue Type: New Feature
>          Components: C++
>    Affects Versions: 2.0.0
>            Reporter: Ben Kietzman
>            Assignee: Ben Kietzman
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 4.0.0
>
>          Time Spent: 5h
>  Remaining Estimate: 0h
>
> RecordBatchProjector should be replaced by an expression calling the "project" compute function.
> Projection currently supports only reordering and subselection of fields, materializing virtual columns where necessary. Replacement with an Expression will enable specifying arbitrary expressions for projected columns:
> {code:java}
> // project an explicit selection:
> // SELECT a as "a", b as "b" ...
> project({field_ref("a"), field_ref("b")}, {"a", "b"});
> // project an arithmetic expression:
> // SELECT a + b as "a + b" ...
> project({add(field_ref("a"), field_ref("b"))}, {"a + b"}){code}
> This will also allow the same expression optimization machinery used for filters to be directly applied to projections. Virtual columns become a consequence of constant folding:
> {code:java}
> // project in a partition where a == 3:
> assert(
>   SimplifyWithGuarantee(
>     project({field_ref("a"), field_ref("b")}, {"a", "b"}),
>     equal(field_ref("a"), literal(3))
>   )
>   == project({literal(3), field_ref("b")}, {"a", "b"})
> ){code}
>  



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