yashmayya opened a new pull request, #18601:
URL: https://github.com/apache/pinot/pull/18601

   ## Description
   
   Flips the default of `pinot.query.mse.skip.pipeline.breaker.stats` from 
`true` to `false`, so multi-stage engine (MSE) pipeline breaker stats are 
included in the `stageStats` field by default.
   
   ## Motivation
   
   This flag was introduced in 1.5.0 with the stats skipped by default, to stay 
conservative while the new stats format rolled out across mixed-version 
clusters. Now that 1.5.0 has been released and the format is understood by all 
instances, it is safe to include these stats by default — exactly the 
transition the flag's Javadoc already anticipated for 1.6.0. Pipeline breaker 
stats shouldn't be skipped by default: they are useful for diagnosing query 
performance, and the leaf-stage pipeline breaker sub-tree is otherwise 
invisible in the returned stage stats.
   
   ## Behavior change
   
   For multi-stage queries that contain a leaf-stage pipeline breaker (e.g. a 
`WHERE col IN (SELECT ...)` semi-join pushed into the leaf stage), the 
`stageStats` JSON now includes the pipeline breaker's sub-tree (a 
`MAILBOX_RECEIVE` → `MAILBOX_SEND` → `LEAF` chain) under the leaf node. 
Consumers that parse `stageStats` will see these additional children. Queries 
without a leaf-stage pipeline breaker are unaffected.
   
   ## Opt-out
   
   Set the cluster config `pinot.query.mse.skip.pipeline.breaker.stats=true` to 
restore the previous behavior. The flag is honored unchanged; only its default 
value moves.
   
   ## Tests
   
   Updated the three pipeline-breaker tests in 
`MultiStageEngineIntegrationTest`:
   - `testPipelineBreakerWithoutKeepingStats` now sets the flag to `true` 
explicitly (it previously relied on the old default) before verifying the leaf 
has no children.
   - All three tests poll for config propagation via `waitForCondition` and 
reset to the new default (`false`) in a `finally` block, making them robust to 
propagation timing and test ordering.
   


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