clintropolis opened a new pull request #12210:
URL: https://github.com/apache/druid/pull/12210


   
   ### Description
   
   Follow up to #12078, adds 
`druid.expressions.homogenizeNullMultiValueStringArrays` with a default value 
of 'false'. If set to true, it opens an escape hatch to a backwards 
compatibility mode for the 'buggy' behavior of some multi-value string array 
functions such as `MV_APPEND` which was fixed in #12078. Prior to that PR, all 
`null`, `[]`, and `[null]` values were treated as `[null]`, which was only 
intended for implicitly mapped single value expression across multi-valued 
string rows, and could produce unintuitive results. This option has been added 
in case anyone happened to be depending on that behavior, but it produces odd 
behavior that impacts all array expressions, so it is not recommended to set 
this flag to true if possible, and instead add a workaround by coercing the 
values explicitly with additional expressions if needed.
   
   
   <hr>
   
   This PR has:
   - [x] been self-reviewed.
   - [ ] added documentation for new or modified features or behaviors.
   - [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, 
ensuring the threshold for [code 
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
 is met.
   - [x] been tested in a test Druid cluster.
   


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