clintropolis opened a new pull request #12194:
URL: https://github.com/apache/druid/pull/12194


   ### Description
   Fixes an issue with `StringAnyAggregatorFactory` which was incorrectly using 
a multi-value dimension selector when `ColumnCapabilities` were unavailable for 
a column, treating it as "unknown", which is correct for the non-vectorized 
engines, but incorrect for vector engines where missing capabilities means that 
the column doesn't exist and so is definitely nulls, which are always 
represented with a single value dimension selector.
   
   This PR fixes the issue and adds additional javadoc and comments to try to 
help avoid making this mistake. Longer term, it might be wise to consider 
moving aggregator factorize methods to using the `ColumnProcessors` factories, 
so that we can re-use the same code for deciding what type of aggregators to 
make for a given input, but I did not do this in this PR.
   
   <hr>
   
   <!-- Check the items by putting "x" in the brackets for the done things. Not 
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   This PR has:
   - [x] 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, 
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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