Support full outer join

[windtalker]

What problem does this PR solve?

Issue Number: close #10777

Problem Summary:

Support FULL OUTER JOIN pushdown to TiFlash for equi-join cases.

What is changed and how it works?

Support full outer join
Translate full outer join note to English

This PR adds TiFlash support for FULL OUTER JOIN in the hash join path with non-empty equi join keys. It wires the DAG join type mapping, nullable schema handling, condition validation, and execution paths needed by FULL OUTER JOIN.

Key changes:

• Add FULL OUTER JOIN mapping from TiDB DAG join type to ASTTableJoin::Kind::Full.
• Make both left-side and right-side output columns nullable for FULL OUTER JOIN.
• Allow FULL OUTER JOIN to carry left/right conditions.
• Fix FULL OUTER JOIN + other condition correctness by using row-flagged map behavior so build-side used marks are applied only after other condition passes.
• Add targeted gtests and an English implementation note.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Support FULL OUTER JOIN pushdown to TiFlash for equi-join cases.

Summary by CodeRabbit

• New Features

  • Added support for full outer joins with unmatched rows, nullable columns, and additional join conditions.
  • Full outer joins without join keys are rejected with a clear error.

• Bug Fixes

  • Improved row preservation, including during spill execution.
  • Corrected nullable output and condition-column handling across join types.

• Documentation

  • Added guidance on full outer join behavior and limitations.

• Tests

  • Added coverage for execution, spill behavior, schema nullability, column pruning, and validation.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 1b75c475-5fff-4d50-9f3b-e6a2c9c1deed

📥 Commits

Reviewing files that changed from the base of the PR and between 2a9bf23 and e6cf853.

📒 Files selected for processing (1)

• dbms/src/Flash/Coprocessor/tests/gtest_join_get_kind_and_build_index.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• dbms/src/Flash/Coprocessor/tests/gtest_join_get_kind_and_build_index.cpp

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📝 Walkthrough

Walkthrough

This PR adds full outer join support to TiFlash. It maps TypeFullOuterJoin, propagates nullable schemas, validates non-equal conditions, preserves unmatched rows through row-flagged execution, and adds tests and documentation.

Changes

Full Outer Join Support

Layer / File(s)	Summary
Join type mapping and validation guards dbms/src/Flash/Coprocessor/DAGUtils.cpp, dbms/src/Flash/Coprocessor/JoinInterpreterHelper.*	Maps TypeFullOuterJoin to ASTTableJoin::Kind::Full, rejects cartesian full joins, names the join type, and permits left/right non-equal conditions.
Nullable schema propagation dbms/src/Flash/Coprocessor/JoinInterpreterHelper.*, dbms/src/Flash/Coprocessor/collectOutputFieldTypes.cpp, dbms/src/Debug/MockExecutor/JoinBinder.cpp	Uses shared nullable-side helpers for join outputs, other-condition columns, field types, and mock executor schemas.
Row-flagged full join execution dbms/src/Interpreters/JoinUtils.h, dbms/src/Interpreters/JoinPartition.cpp, dbms/src/Interpreters/Join.cpp, dbms/src/DataStreams/ScanHashMapAfterProbeBlockInputStream.cpp	Adds full-join row-flagged probing, unmatched-row handling, null padding, used-entry guards, and post-probe scanning.
Tests and design documentation dbms/src/Flash/Coprocessor/tests/*, dbms/src/Flash/tests/*, dbms/src/TestUtils/tests/*, docs/note/fullouter_join.md	Adds coverage for mapping, nullability, conditions, execution, column pruning, spilling, and mock schemas. The design note records the supported scope and implementation checklist.

Estimated code review effort: 4 (Complex) | ~75 minutes

Poem

A rabbit checks each join-side row,
Full outer paths now safely flow.
Nulls appear where unmatched rows land,
Row-flagged maps keep results planned.
Tests and notes mark every way—
Hop, hop, hooray for join support! 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: adding FULL OUTER JOIN support.
Description check	✅ Passed	The description covers the problem, implementation, tests, side effects, documentation, compatibility, and release note requirements.
Linked Issues check	✅ Passed	The changes address all linked issue objectives, including plumbing, nullable schemas, cartesian-join guarding, conditions, execution fixes, tests, and documentation.
Out of Scope Changes check	✅ Passed	The code, tests, and implementation note are directly related to FULL OUTER JOIN support and the linked issue scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[windtalker]

/run-check-issue-triage-complete

[windtalker]

/run pull-integration-test

[windtalker]

/test pull-unit-test

[windtalker]

/test pull-integration-test