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https://issues.apache.org/jira/browse/CALCITE-4879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17440074#comment-17440074
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Jacques Nadeau commented on CALCITE-4879:
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I suggest we deprecate constructors in 1.29 and then move to abstract as part
of 1.30
> Make RelMetadataQuery abstract
> ------------------------------
>
> Key: CALCITE-4879
> URL: https://issues.apache.org/jira/browse/CALCITE-4879
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Jacques Nadeau
> Priority: Major
>
> The RelOptCluster.setMetadataQuerySupplier() and RelMedataQuery abstraction
> are great at separating how planners and rules consume metadata versus how
> metadata is produced. While details about how metadata is produced is
> (mostly) not leaked in the api of RelMetadataQuery, the class does assume
> that metadata will be produced via the current mechanisms surrounding
> RelMetadataProviders and MetadataHandlers. This ticket targets separating the
> production of metadata from the consumption interface, by making
> RelMetadataQuery abstract (either as an abstract class or as a interface) and
> moving the handler and provider specific implementations to an implementation
> of RelMetadataQuery. This will allow a broader breadth of experimentation to
> be undertaken. For example, one example people have been evaluating is
> whether a lambda based system would be easier to understand and debug, as
> performant and more AOT friendly than the existing systems of chains and
> janino compilation.
> To accomplish this task, the first step will be to deprecate the existing
> constructors and inform people to use a concrete subtype. Once deprecated,
> the actual logic that currently exists in RelMetadataQuery can be extracted
> into the concrete subtype and the base class can be made either abstract or
> an interface (depending on what seems most appropriate at the time).
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