[CALCITE-7439] Qualify GROUP BY keys for DISTINCT over joins

[bvolpato]

Summary

RelToSqlConverter emitted ambiguous GROUP BY keys after semi-join rewrites for DISTINCT queries over joins with merged key columns.

This PR qualifies merged join keys when building GROUP BY and corresponding SELECT keys from a join-shaped aggregate input:

• LEFT / INNER-like joins qualify against the left alias
• RIGHT joins qualify against the right alias
• FULL joins use COALESCE(left.key, right.key)
• fallback detection also covers same-name = predicates, not just USING

Reproduction query

WITH product_keys AS (
  SELECT p."product_id",
         (SELECT MAX(p3."product_id")
          FROM "foodmart"."product" p3
          WHERE p3."product_id" = p."product_id") AS "mx"
  FROM "foodmart"."product" p
)
SELECT DISTINCT pk."product_id"
FROM product_keys pk
LEFT JOIN "foodmart"."product" p2 USING ("product_id")
WHERE pk."product_id" IN (
  SELECT p4."product_id"
  FROM "foodmart"."product" p4
)

Error before this PR

org.apache.calcite.runtime.CalciteContextException:
From line 12, column 10 to line 12, column 21: Column 'product_id' is ambiguous

Tests

• Added testPostgresqlRoundTripDistinctLeftJoinInSubqueryWithSemiJoinRules
• Added testPostgresqlRoundTripDistinctLeftJoinUsingTwoKeysWithSemiJoinRules
• Added assertions for RIGHT and FULL join qualification behavior
• Re-parse and validate the generated PostgreSQL to catch ambiguous output
• Ran the focused RelToSqlConverterTest regression slice locally on Java 21

Disclaimer

The changes were done with the assistance of Codex in response to an actual bug when using Calcite in production. The changes were manually verified prior to sending this to review.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
65.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[caicancai]

I will conduct the review over the weekend.

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 90 days if no further activity occurs. If you think that’s incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@calcite.apache.org list. Thank you for your contributions.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
65.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[bvolpato]

[review-prs-verification]
Head: 9c84517
Ready to merge: yes
Files reviewed.

• core/src/main/java/org/apache/calcite/rel/rel2sql/RelToSqlConverter.java
• core/src/test/java/org/apache/calcite/rel/rel2sql/RelToSqlConverterTest.java
  Tests/checks run and results.
• Local: ./gradlew --no-daemon :core:checkstyleMain :core:checkstyleTest :core:compileJava :core:compileTestJava :core:test --tests org.apache.calcite.rel.rel2sql.RelToSqlConverterTest (PASS, BUILD SUCCESSFUL)
• GitHub CI on head 9c84517: all listed checks PASS; Linux (JDK 8) Slow Tests SKIPPED by workflow.
  Fixes committed/pushed, if any.
• None.
  Final CI status.
• PASS (no failing checks reported).

[mihaibudiu]

SqlIdentifier has a method isSimple

[mihaibudiu]

can you pull this into a helper function to avoid repeating it?

[mihaibudiu]

although these methods are private having some JavaDoc would help maintain them in the future and review them now.

[mihaibudiu]

I wonder what happens if you have a ROW field and in the join you use a qualified field deeper than 2. Maybe you can add a test case for that?