[akasakariko]

Fixes #20819

Description

Return a FieldAccessPostAggregator when TDIGEST_GENERATE_SKETCH reuses an existing sketch, so selecting TDIGEST_QUANTILE first no longer registers the same aggregator twice

Add a SQL regression that checks the quantile and serialized sketch results and verifies that both outputs share one native sketch aggregator

Release note

Fixed SQL planning failures when TDIGEST_GENERATE_SKETCH follows TDIGEST_QUANTILE with the same input and compression

Validation

Tested with Temurin 25.0.5 and Maven 3.9.11

• The new SQL regression fails before the fix with Duplicate field name: a0:agg and passes after the fix
• mvn -B -ntp test -pl extensions-contrib/tdigestsketch -Dsurefire.rerunFailingTestsCount=0 -Pskip-static-checks -Dweb.console.skip=true: 57 tests passed, no failures or skips
• mvn -B -ntp verify -pl extensions-contrib/tdigestsketch -DskipTests -Dweb.console.skip=true -Dmaven.javadoc.skip=true -Dcyclonedx.skip=true: passed with static checks enabled
• mvn -B -ntp -pl extensions-contrib/tdigestsketch -Prat apache-rat:check dependency:analyze-only spotbugs:check -DoutputXML=true -DignoreNonCompile=true -DfailOnWarning=true: passed
• javap -public confirms unchanged public signatures for TDigestGenerateSketchSqlAggregator

This PR has:

• been self-reviewed
• a release note entry in the PR description
• added a regression test

[FrankChen021]

🟢 Approval recommended

No actionable issues found in this review. The reuse path correctly references the existing T-Digest aggregator through a field-access post-aggregator, and the regression test covers quantile-first planning and serialized sketch output.

Reviewed 2 of 2 changed files.

Validation: git diff --check passed; tests and builds were not run during this static review.

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This is an automated review by Codex GPT-5.6-Luna(max)