[jira] [Commented] (CALCITE-4942) Deprecate boilerplate for Rel Metadata

["Jacques Nadeau (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-4942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17465853#comment-17465853 ] 

Jacques Nadeau commented on CALCITE-4942:
-----------------------------------------

[~jamesstarr], I actually did several iterations on how to construct handlers to be cleaner/more modern/simpler. The best I came up with can be found in [this gist|https://gist.github.com/jacques-n/19e06a39704d789a06cdd06cffacc8c1].

There are two different things that I think are ideal to accomplish:
- handlers are generic and declare the relnode interface they operate on. This creates an explicit functional definition of what metadata methods should look like (something that only exists implicitly today).
- handlers have a generic method for metadata and use a generic definition. This allows a simplification of code that wants to invoke them since there are only 6 total functional signatures that exist across all metadata operations. (It's six to avoid boxing, otherwise it would be three.)

If we're doing breaking changes, I think it would be helpful to introduce these patterns to not only clean up the old stuff but introducing a more standardized way to express these things.

> Deprecate boilerplate for Rel Metadata
> --------------------------------------
>
>                 Key: CALCITE-4942
>                 URL: https://issues.apache.org/jira/browse/CALCITE-4942
>             Project: Calcite
>          Issue Type: Improvement
>            Reporter: James Starr
>            Assignee: James Starr
>            Priority: Major
>              Labels: pull-request-available
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> To define a Rel metadata type you need to create 2 class, 2 methods and a static variable.  However, current implementation of RelMetadata back by Janino only require the Handler class a single method signature.  The current metadata handler interface also has a generic that provides no type safety through out code. 
> Currently to define a rel metadata type:
> {code:java}
> public class MyMetadata extends Metadata {
>   MetadataDef DEF = ...
>  
>  VALUE myMethod1();
>   class Handler extends RelHandler<MyMetadata> {
>     VALUE myMethod2(RelNode, MetadataQuery)
>   }
> }
> {code}
> What is actually needed to define a rel metadata type:
> {code:java}
> class MyMetadataHandler extends RelHandler {
>   VALUE myMethod(RelNode, MetadataQuery)
> }
> {code}



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