xiangfu0 commented on PR #18103:
URL: https://github.com/apache/pinot/pull/18103#issuecomment-4227506269

   Follow-up on the split reproducer PR: I updated #18166 so it now includes a 
direct `3.2` vs `3.3` comparison, not just the `3.3`-only merge-order 
reproducer.
   
   What changed:
   - `TDigestVersionComparisonTest` loads `t-digest:3.2` and `t-digest:3.3` 
side-by-side in isolated classloaders and runs the same deterministic 
Pinot-like hierarchical merge dataset through both versions.
   - `TDigestMergeOrderReproducerTest` remains the pure `3.3` merge-order 
sensitivity check.
   
   So the answer to “is this a test that passes on 3.2 but fails on 3.3?” is:
   - the original merge-order test is `3.3`-only
   - the new comparison test is the direct A/B check between `3.2` and `3.3`
   
   Repro:
   ```bash
   ./mvnw -pl pinot-segment-local 
-Dtest=TDigestVersionComparisonTest,TDigestMergeOrderReproducerTest 
-Dsurefire.failIfNoSpecifiedTests=false test
   ```
   
   The comparison test asserts that, on the minimized exact-quantile scenario:
   - `3.2 @ compression 100` stays below `0.0002` max normalized error
   - `3.3 @ compression 100` is above `0.004` max normalized error and more 
than `20x` worse than `3.2`
   - `3.3 @ compression 150` recovers below `0.0002`
   
   From the local exploration used to lock in that scenario, the observed 
numbers were roughly:
   - `3.2 @ 100`: `0.000074` max normalized error with `121` centroids
   - `3.3 @ 100`: `0.005094` max normalized error with `55` centroids
   - `3.3 @ 150`: `0.000049` max normalized error with `79` centroids
   
   I also reran the repro command in 10 fresh Surefire JVMs and it passed 
`10/10`.
   


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