GH-30894: [C++][Python] Fix Table::Slice returning incorrect length for tables with no columns

Author: AdvancedUno

cpp/src/arrow/table.cc

@@ -101,7 +101,7 @@ class SimpleTable : public Table {
 
   std::shared_ptr<Table> Slice(int64_t offset, int64_t length) const override {
     auto sliced = columns_;
-    int64_t num_rows = length;
+    int64_t num_rows = std::max<int64_t>(0, std::min(length, this->num_rows() - offset));
     for (auto& column : sliced) {
       column = column->Slice(offset, length);
       num_rows = column->length();

cpp/src/arrow/table_test.cc

@@ -1319,6 +1319,23 @@ TEST_F(TestTable, Slice) {
                     *three->Slice(length + length / 3, 2 * (length - length / 3)));
 }
 
+TEST_F(TestTable, SliceZeroColumns) {
+  const int64_t length = 10;
+  auto empty_columns_table = Table::Make(::arrow::schema({}),std::vector<std::shared_ptr<ChunkedArray>>{}, length);
+  ASSERT_EQ(empty_columns_table->num_rows(), length);
+  ASSERT_EQ(empty_columns_table->Slice(3)->num_rows(), length - 3);
+  ASSERT_EQ(empty_columns_table->Slice(0)->num_rows(), length);
+  ASSERT_EQ(empty_columns_table->Slice(3, 100)->num_rows(), length - 3);
+  ASSERT_EQ(empty_columns_table->Slice(3, 4)->num_rows(), 4);
+  ASSERT_EQ(empty_columns_table->Slice(length + 5)->num_rows(), 0);
+
+  MakeExample1(length);
+  auto table = Table::Make(schema_, columns_);
+  ASSERT_OK_AND_ASSIGN(auto no_columns, table->SelectColumns({}));
+  ASSERT_EQ(no_columns->Slice(1)->num_rows(), table->Slice(1)->num_rows());
+  ASSERT_EQ(no_columns->Slice(1, 4)->num_rows(), 4);
+}
+
 TEST_F(TestTable, RemoveColumn) {
   const int64_t length = 10;
   MakeExample1(length);

python/pyarrow/tests/test_table.py

@@ -1361,6 +1361,22 @@ def test_table_slice_getitem():
     return _table_like_slice_tests(pa.table)
 
 
+def test_table_slice_no_columns():
+    table = pa.table({'col': range(3)})
+    empty = table.select([])
+    assert empty.num_rows == 3
+
+    # slice(offset) clamps to the remaining rows
+    assert empty.slice(1).num_rows == 2
+    assert empty.slice(0).num_rows == 3
+    # slice(offset, length) must not exceed the remaining rows
+    assert empty.slice(1, 4).num_rows == 2
+    # consistent with __getitem__ slicing, which already worked
+    assert empty[1:].num_rows == 2
+    # offset past the end yields an empty table
+    assert empty.slice(5).num_rows == 0
+
+
 @pytest.mark.pandas
 def test_slice_zero_length_table():
     # ARROW-7907: a segfault on this code was fixed after 0.16.0