GH-37643: [C++] Enhance arrow::Datum::ToString

[mapleFU]

Rationale for this change

Add print child fields for arrow::Datum::ToString. Because previously it just print a type.

What changes are included in this PR?

Add detail in arrow::Datum::ToString

Are these changes tested?

no

Are there any user-facing changes?

Yes

• Closes: [C++] Enhance arrow::Datum::ToString #37643

[github-actions]

⚠️ GitHub issue #37643 has been automatically assigned in GitHub to PR creator.

[mapleFU]

make_array will generate a temp ::arrow::Array object, don't know if we have a better way for this

[bkietz]

Unfortunately I don't think so. It won't be an O(N) copy or anything though, so not the end of the world.

[mapleFU]

/arrow/c_glib/test/test-table-datum.rb:52:in `test_to_string'
     49:   end
     50: 
     51:   def test_to_string
  => 52:     assert_equal("Table", @datum.to_s)
     53:   end
     54: 
     55:   def test_value
<"Table"> expected but was
<"Table(visible: bool\n" +
"----\n" +
"visible:\n" +
"  [\n" +
"    [\n" +
"      true,\n" +
"      false\n" +
"    ]\n" +
"  ]\n" +
")">

Seems this should also be fixed lol

[mapleFU]

@bkietz @kou Would you mind take a look?

[bkietz]

LGTM, thanks!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit d9ee3a7.

There was 1 benchmark result indicating a performance regression:

• Commit Run on ursa-i9-9960x at 2023-09-15 23:36:05Z
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-20, scale_factor=1

The full Conbench report has more details.