[query-runtime] Skip leaf execution for empty dynamic filter

[wirybeaver]

PR flow is maintained manually as ASCII in the Design section below.

Summary

This PR replaces the unused partition-coalescing policy with a minimal, executable AQE proof of concept in Pinot's multi-stage engine.

For a dynamic-broadcast SEMI JOIN, the existing pipeline breaker materializes the build side before the probe leaf is compiled. When that runtime result is empty, Pinot now skips constructing and executing the probe-side single-stage requests while preserving the normal multi-stage operator chain, mailbox EOS, pipeline-breaker statistics, and leaf/send statistics.

This is deliberately narrower than changing stage parallelism: Pinot's current streaming mailbox graph is eagerly dispatched and binds routing to concrete workers, so resizing a downstream stage after its producer completes is not yet safe.

Design

build-side mailbox exchange (PIPELINE_BREAKER)
                      |
                      v
          materialize build rows + stats
                      |
                 build empty?
                 /          \
              yes            no
               |              |
      skip probe SSE      construct 1-2 SSE
      request creation      leaf requests
               |              |
               +-------+------+
                       |
                       v
             normal LeafOperator path
                       |
                       v
          mailbox EOS + leaf/send/build stats

The skip decision is explicit: normal LeafOperator construction still requires one or two server requests. Empty-input-sensitive operators—global aggregates and grouping sets containing the grand-total () set—remain on the normal SSE path. EXPLAIN also retains normal request construction.

Roadmap: downstream parallelism AQE

Supporting "complete a stage, then resize an undispatched downstream stage" requires all three seams together:

producer stage
      |
      v
materialized logical partitions (consumer-independent identities)
      |
      v
all-worker completion barrier + exact partition bytes
      |
      v
broker AQE rule: coalesce M logical partitions into K ranges
      |
      v
late-bind ranges to K workers and dispatch consumer stage

1. Add an opt-in materialized HASH exchange with static parallelism.
2. Add broker-owned staged dispatch and all-worker completion barriers, initially without adaptation.
3. Late-bind materialized logical partitions to consumer workers.
4. Add deterministic contiguous partition coalescing (M -> K) using completed byte statistics.
5. Harden with stage-attempt fencing, cleanup/quotas, observability, rollout gating, and mixed-version behavior.

Join reordering, dynamic stage insertion/removal, running-stage cancellation, and retries are out of scope for the first rule.

Test Plan

• 37 focused LeafOperator, pipeline-breaker, and server-plan tests passed.
• 141 QueryRunnerTest cases passed, including empty-build global aggregation and dynamic-broadcast success/failure paths.
• ./mvnw spotless:apply -pl pinot-query-runtime
• ./mvnw checkstyle:check -pl pinot-query-runtime
• ./mvnw license:format -pl pinot-query-runtime
• ./mvnw license:check -pl pinot-query-runtime
• ./mvnw test-compile -pl pinot-query-runtime -Dmaven.compiler.showDeprecation=true -Dmaven.compiler.showWarnings=true

A full dependency build was also attempted but is currently blocked by the existing pinot-segment-local ZstandardDecompressor / org.jetbrains.annotations.NotNull compilation issue.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.24%. Comparing base (7a00b23) to head (d36a227).

Files with missing lines	Patch %	Lines
...che/pinot/query/runtime/operator/LeafOperator.java	81.81%	0 Missing and 2 partials ⚠️
...ry/runtime/plan/server/ServerPlanRequestUtils.java	66.66%	0 Missing and 1 partial ⚠️
.../runtime/plan/server/ServerPlanRequestVisitor.java	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #19470       +/-   ##
=============================================
+ Coverage     39.94%   68.24%   +28.30%     
- Complexity     1449     1450        +1     
=============================================
  Files          3520     3520               
  Lines        228940   228951       +11     
  Branches      36313    36317        +4     
=============================================
+ Hits          91453   156253    +64800     
+ Misses       129215    60444    -68771     
- Partials       8272    12254     +3982     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	68.24% <88.88%> (+28.30%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	68.24% <88.88%> (+28.30%)	⬆️
unittests	68.24% <88.88%> (+28.30%)	⬆️
unittests1	58.14% <88.88%> (?)
unittests2	39.94% <0.00%> (-0.01%)	⬇️

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