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https://issues.apache.org/jira/browse/CALCITE-4539?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17303622#comment-17303622
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James Starr commented on CALCITE-4539:
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This would allow for custom behavior/validation to be done in client code bases.
> Support pluggable metadata handlers and caching
> -----------------------------------------------
>
> Key: CALCITE-4539
> URL: https://issues.apache.org/jira/browse/CALCITE-4539
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: James Starr
> Priority: Major
>
> Calcite janino backed metadata does not support all the functionality that
> the java reflection backed metadata system, such as customizing the cache to
> use soft refs or have a max size. Furthermore, the janino backed metadata
> provider has many opinion that other are not universally shared, such as lazy
> loading, requiring all rels to registered to a global singleton, use of
> thread locals. There is no easy way to currently override these opinions.
> Calcites metadata system should consist of 3 APIs: metadata consumer,
> metadata implementors, metadata behavior. The metadata consumer need to able
> to fetch the appropriate metadata and invalidate the cache. Currently this
> is all done through RelMetadataQuery with some very leaky abstractions.
> Metadata implementors create functions that extract the metadata for
> particular node types. These functions are registered via
> ReflectiveRelMetadataProvider and ChainedRelMetadataProvider. Attempting to
> configure the internal behavior does not currently have an api. Currently
> porptions of it a scattered through out various class and are frequently not
> extensible.
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