fix(transformer): preserve existing BYTES value when transform yields null

[xiangfu0]

Summary

• Fix ExpressionTransformer overriding an existing non-null byte[] (BYTES column) with null when a transform expression evaluates to null. byte[] is logically a scalar in Pinot, but it accidentally falls into the isArray() branch intended for nested fields, so the null-result path called applyTransformedValue(record, column, null) and cleared the BYTES column.
• Extract a shouldPreserveExistingValue helper that skips the apply step when the transform yields null AND an existing non-null value is present, for two cases:
  1. Implicit map transforms (mapField__KEYS/__VALUES) — generalizes the prior guard introduced in Add post-partial-upsert transforms for derived columns #17308 so it also covers the shouldApplyTransform branch (e.g. columns flagged null via putDefaultNullValue).
  2. BYTES single-value columns — treat byte[] as the scalar it represents.
• Guard applies in both the populate-missing/overwrite branch and the existing array/Collection/Map branch, including the post-upsert overwrite path (_overwriteExistingValues=true).

Related: #17308.

Test plan

• Added testImplicitMapTransformDoesNotOverrideExistingBytesValueWhenSourceMissing — implicit map + BYTES with default-null marker
• Added testTransformReturningNullDoesNotOverrideExistingBytesValue — explicit transform yielding null, regular ingestion path
• Added testPostUpsertTransformReturningNullDoesNotOverrideExistingBytesValue — explicit transform yielding null, post-upsert overwrite path
• All 18 tests in ExpressionTransformerTest pass
• ./mvnw spotless:apply license:format checkstyle:check license:check -pl pinot-segment-local clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes a null-handling edge case in ExpressionTransformer where BYTES single-value columns (represented as byte[]) were incorrectly treated as “nested/array” fields and could be cleared when a transform expression evaluated to null. The change ensures existing non-null byte[] values are preserved when the transform yields null, including in the post-upsert overwrite mode.

Changes:

• Add a shouldPreserveExistingValue(...) helper to skip applying a null transform result when an existing non-null value must be retained (implicit MAP-derived columns and BYTES byte[]).
• Apply the preservation guard in both the “shouldApplyTransform” path and the “array/collection/map compatibility” path.
• Add targeted unit tests covering implicit MAP + BYTES, explicit null transforms on BYTES, and post-upsert overwrite mode.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/recordtransformer/ExpressionTransformer.java	Prevents null transform results from clobbering existing non-null BYTES (byte[]) values; reuses the same guard for implicit MAP-derived columns.
pinot-segment-local/src/test/java/org/apache/pinot/segment/local/recordtransformer/ExpressionTransformerTest.java	Adds regression tests for BYTES preservation behavior in normal ingestion and post-upsert overwrite paths.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.69%. Comparing base (4863cdc) to head (4a9d066).
⚠️ Report is 37 commits behind head on master.

Files with missing lines	Patch %	Lines
...local/recordtransformer/ExpressionTransformer.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18503      +/-   ##
============================================
+ Coverage     63.65%   63.69%   +0.04%     
+ Complexity     1735     1684      -51     
============================================
  Files          3254     3266      +12     
  Lines        199446   199898     +452     
  Branches      30977    31048      +71     
============================================
+ Hits         126953   127333     +380     
- Misses        62356    62403      +47     
- Partials      10137    10162      +25     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	63.69% <0.00%> (+0.04%)	⬆️
temurin	63.69% <0.00%> (+0.04%)	⬆️
unittests	63.69% <0.00%> (+0.04%)	⬆️
unittests1	55.81% <0.00%> (+0.10%)	⬆️
unittests2	34.96% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.