[grevend]

Which issue does this PR close?

• Closes MIN/MAX don't unwrap nested encodings inside Dictionary columns #26290.

Rationale for this change

MIN/MAX fail with an internal error on Dictionary<_, RunEndEncoded<_, _>> columns, and return a Dictionary type instead of the value type for Dictionary<_, Dictionary<_, _>> columns. See #26290.

What changes are included in this PR?

The Dictionary arm of get_min_max_result_type now recurses into its value type, the same as the RunEndEncoded arm added in #26248.

What is the testing strategy for this PR?

• test_get_min_max_return_type_coerce_dictionary covers Dictionary<Int32, RunEndEncoded<Int32, UInt32>> → UInt32 and Dictionary<Int32, Dictionary<Int32, UInt32>> → UInt32.
• New cases in dictionary.slt run MIN/MAX and arrow_typeof(MAX(...)) over both column types, one query per type.

Without the fix, the Dictionary<_, RunEndEncoded<_, _>> query fails with the internal error, and the Dictionary<_, Dictionary<_, _>> query returns a Dictionary type.

Are there any user-facing changes?

MIN and MAX over a Dictionary column whose values are encoded now return the plain value type. There are no API changes.

Generated-by: Claude Code (Claude Opus 5.5). I reviewed the change and can answer for it.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.85%. Comparing base (c52eb97) to head (ca9988f).

Files with missing lines	Patch %	Lines
datafusion/functions-aggregate/src/min_max.rs	83.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26371      +/-   ##
==========================================
- Coverage   82.85%   82.85%   -0.01%     
==========================================
  Files        1151     1151              
  Lines      455996   456007      +11     
  Branches   455996   456007      +11     
==========================================
+ Hits       377805   377811       +6     
+ Misses      55239    55238       -1     
- Partials    22952    22958       +6     

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[grevend]

@Jefffrey this is the follow-up you suggested on #26248: the Dictionary arm now recurses like the RunEndEncoded one. Would you have time to take a look?

[Rich-T-kid]

Looks good to me! thanks @grevend

[Jefffrey]

thanks