jon-wei opened a new pull request #10689:
URL: https://github.com/apache/druid/pull/10689


   This PR introduces a new tuning config parameter, `maxColumnsToMerge`. 
   
   This functions as a limit on how many segments can be merged at the same 
time by the IndexMerger. When the column limit is exceeded across a set of 
segments, the IndexMerger will break the segments to be merged into smaller 
phases, and merge the smaller phases in a tree. 
   
   A minimum of 2 segments will be merged at once, regardless of the limit. If 
there is only 1 segment being merged, the limit does not apply. (A warning is 
logged in these cases, but merging is allowed to proceed).
   
   Currently only the native batch and parallel ingest task tuning config have 
this new parameter added, this PR does not add support for it to Kafka/Kinesis 
ingestion yet.
   
   This PR has:
   - [x] been self-reviewed.
   - [ ] added documentation for new or modified features or behaviors.
   - [ ] added Javadocs for most classes and all non-trivial methods. Linked 
related entities via Javadoc links.
   - [ ] added or updated version, license, or notice information in 
[licenses.yaml](https://github.com/apache/druid/blob/master/licenses.yaml)
   - [ ] 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.
   - [ ] added integration tests.
   - [ ] been tested in a test Druid cluster.


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