GH-30894: [C++][Python] Fix Table::Slice returning incorrect length f…

[AdvancedUno]

Rationale for this change

Table::Slice on a table with no columns does not adjust the row count. SimpleTable::Slice computes num_rows inside the per-column loop, so with zero columns the loop never runs and num_rows keeps the raw length argument instead of being clamped to the rows actually available.

import pyarrow as pa
table = pa.table({'col': range(3)})

table.slice(1).num_rows               # 2  (correct)
table.select([])[1:].num_rows         # 2  (correct - __getitem__ normalizes)
table.select([]).slice(1).num_rows    # 3  (WRONG, should be 2)
table.select([]).slice(1, 4).num_rows # 4  (WRONG, should be 2)

This violates the contract documented in table.h: "If there are not enough rows in the table, the length will be adjusted accordingly."

What changes are included in this PR?

• cpp/src/arrow/table.cc : in SimpleTable::Slice, seed num_rows with a clamped value, std::max<int64_t>(0, std::min(length, num_rows() - offset)), so it is correct even when the column vector is empty. When columns exist the loop still overwrites num_rows with column->length(), so behavior for tables with columns is unchanged. The single-arg Table::Slice(offset) routes through this method, so it is fixed as well.
• cpp/src/arrow/table_test.cc: new TEST_F(TestTable, SliceZeroColumns).
• python/pyarrow/tests/test_table.py: new test_table_slice_no_columns

Are these changes tested?

Yes. The new C++ and Python regression tests cover slicing a zero-column table with offset only, offset + length, an offset past the end , and consistency with a table whose columns were all removed using SelectColumns({}).

Are there any user-facing changes?

Yes. Table.slice() on a table with no columns now returns the correct num_rows instead of echoing back the raw length argument. This is a bug fix, there are no API changes.

Closes #30894

• GitHub Issue: [C++][Python] Slicing a table with no columns returns a table with incorrect length. #30894

[github-actions]

⚠️ GitHub issue #30894 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #30894 has been automatically assigned in GitHub to PR creator.

[thisisnic]

Thanks for the PR @AdvancedUno! The linter is failing - you can find instructions here on running the linter, but let me know if you have any issues!

[AdvancedUno]

Hi @thisisnic, I just made the fix!