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https://issues.apache.org/jira/browse/ARROW-10322?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Neal Richardson resolved ARROW-10322.
-------------------------------------
Resolution: Fixed
Issue resolved by pull request 8894
[https://github.com/apache/arrow/pull/8894]
> [C++][Dataset] Minimize Expression to a wrapper around compute::Function
> ------------------------------------------------------------------------
>
> Key: ARROW-10322
> URL: https://issues.apache.org/jira/browse/ARROW-10322
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Affects Versions: 1.0.1
> Reporter: Ben Kietzman
> Assignee: Ben Kietzman
> Priority: Major
> Labels: pull-request-available
> Fix For: 3.0.0
>
> Time Spent: 17.5h
> Remaining Estimate: 0h
>
> The Expression class hierarchy was originally intended to provide generic,
> structured representations of compute functionality. On the former point they
> have been superseded by compute::
> {Function, Kernel, ...}
> which encapsulates validation and execution. In light of this Expression can
> be drastically simplified and improved by composition with these classes.
> Each responsibility which can be deferred implies less boilerplate when
> exposing a new compute function for use in datasets. Ideally any compute
> function will be immediately available to use in a filter or projection.
> {code:java}
> struct Expression {
> using Literal = std::shared_ptr<Scalar>;
> struct Call {
> std::shared_ptr<ScalarFunction> function;
> std::shared_ptr<FunctionOptions> options;
> std::vector<Expression> arguments;
> };
> util::variant<Literal, FieldRef, Call> value;
> };
> {code}
> A simple discriminated union as above should be sufficient to represent
> arbitrary filters and projections: any expression which results in type
> {{bool}} is a valid filter, and any expression which is a {{Projection}} may
> be used to map one record batch to another.
> Expression simplification (currently implemented in {{Expression::Assume}})
> is an optimization used for example in predicate pushdown, and therefore need
> not exhaustively cover the full space of available compute functions.
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