[GitHub] [incubator-druid] clintropolis opened a new pull request #8019: multi-value string expression transformation fix

[GitBox <gi...@apache.org>]

clintropolis opened a new pull request #8019: multi-value string expression transformation fix
URL: https://github.com/apache/incubator-druid/pull/8019
 
 
   ### Description
   
   This PR fixes an issue with the automatic translation of expressions for multi-value string column inputs, where if an identifier was duplicated, the expression would not transform correctly.
   
   This has been resolved by modifying expression parsing to uniquely identify all _non lambda bound_ `IdentifierExpr`, at parsing transformation time in `ExprListenerImpl`. `LambdaExpr` identifiers are marked on entering a lambda expression, and all non-lambda identifiers are given a unique identifier to accompany the identifier to use to get the value from the `Expr.ObjectBinding`.
   
   `Expr.BindingDetails` has been reworked to accommodate this change, as well as simplified. Further refactoring took place in `Parser` to handle the transformation required to create `LambdaExpr` binding `IdentifierExpr` to ensure the correct transformation.
   
   Example:
   before patch:
   ```
   concat(x, '-lol-', x)
   ``` 
   
   where `x` is a multi-value string dimension would result in a transformation to something like 
   
   ```
   map((x) -> concat(x, '-lol-', x), x)
   ```
    
   after patch:
   this will correctly map to something like 
   ```
   cartesian_map((x, x_0) -> concat(x, '-lol-', x_0), x, x_0)
   ```
   
   <hr>
   
   This PR has:
   - [ ] been self-reviewed.
   - [x] added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
   - [x] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader. 
   - [x] added unit tests or modified existing tests to cover new code paths.
   

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