test: add functional_dependencies.slt covering functional dependency driven optimizations

[alamb]

Which issue does this PR close?

This PR adds test coverage rather than closing an issue. It documents the current behavior of these bugs so that fixing them shows up as a test change:

• Incorrect results with unique constraints #23634
• Incorrect results: ORDER BY drops a sort key based on a nullable UNIQUE constraint #23818
• Incorrect results: GROUP BY column dropped based on a nullable UNIQUE constraint #23819
• Incorrect results: GROUP BY returns two rows for the NULL group with a nullable UNIQUE constraint #23820

Rationale for this change

While reviewing #23636 @neilconway and I kept coming up with more examples of bad plans, and it was not obvious which of the surrounding wrong answers were pre-existing and which the PR introduced.

What changes are included in this PR?

A new datafusion/sqllogictest/test_files/functional_dependencies.slt with one section per consumer of functional dependencies:

1. ReplaceDistinctWithAggregate — removing DISTINCT
2. eliminate_duplicated_expr — dropping trailing ORDER BY keys
3. optimize_projections — dropping GROUP BY expressions
4. add_group_by_exprs_from_dependencies — selecting non-grouped columns
5. GROUP BY derived keys on the NULL-padded side of an outer join

Cases that currently return wrong answers are labelled BUG with the expected
result and a link to the issue:

Case	Symptom	Issue
1.2	DISTINCT over a nullable UNIQUE column returns both NULL rows	#23634
2.2	ORDER BY x, y drops the y key, so the NULL rows come back unordered	#23818
3.2	GROUP BY x, y drops y, merging the two NULL groups and losing a row	#23819
4.2	SELECT x, y ... GROUP BY x returns two rows for the x = NULL group	#23820

Are these changes tested?

CI

Are there any user-facing changes?

No.

[alamb]

It would be nice to add coverage for multi-column functional dependencies too, but I wanted to keep this PR small to make it eaiser to review

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.71%. Comparing base (582453b) to head (8fdf2e0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23821      +/-   ##
==========================================
- Coverage   80.72%   80.71%   -0.01%     
==========================================
  Files        1090     1090              
  Lines      370384   370384              
  Branches   370384   370384              
==========================================
- Hits       298975   298950      -25     
- Misses      53590    53610      +20     
- Partials    17819    17824       +5     

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[comphead]

Thanks @alamb, maybe we can make comments little bit less verbose

[alamb]

Thanks @alamb, maybe we can make comments little bit less verbose

Thanks @comphead -- I actually cut down the comments quite a bit from the initial LLM output -- the current set I think explains what is going on. Given these are all bugs related to NULL corner cases (for example, I learned that UNIQUE allows duplicate NULL values!) I wanted to try and help future readers understand them if they started failnig

[alamb]

Thanks for the review @comphead -- I took another pass and deleted a bunchmore commentary in 8fdf2e0

[alamb]

Only logical plans are shown, since all of these rules run during logical optimization.