Revert "UNPICK"

This reverts commit aba97dd.

Author: kosiew

docs/source/user-guide/dataframe/index.rst

@@ -145,10 +145,39 @@ To materialize the results of your DataFrame operations:
     
     # Display results
     df.show()                         # Print tabular format to console
-    
+
     # Count rows
     count = df.count()
 
+PyArrow Streaming
+-----------------
+
+DataFusion DataFrames implement the ``__arrow_c_stream__`` protocol, enabling
+zero-copy streaming into libraries like `PyArrow <https://arrow.apache.org/>`_.
+Earlier versions eagerly converted the entire DataFrame when exporting to
+PyArrow, which could exhaust memory on large datasets. With streaming, batches
+are produced lazily so you can process arbitrarily large results without
+out-of-memory errors.
+
+.. code-block:: python
+
+    import pyarrow as pa
+
+    # Create a PyArrow RecordBatchReader without materializing all batches
+    reader = pa.RecordBatchReader._import_from_c_capsule(df.__arrow_c_stream__())
+    for batch in reader:
+        ...  # process each batch as it is produced
+
+DataFrames are also iterable, yielding :class:`pyarrow.RecordBatch` objects
+lazily so you can loop over results directly:
+
+.. code-block:: python
+
+    for batch in df:
+        ...  # process each batch as it is produced
+
+See :doc:`../io/arrow` for additional details on the Arrow interface.
+
 HTML Rendering
 --------------
 

python/datafusion/dataframe.py

@@ -26,6 +26,7 @@
     TYPE_CHECKING,
     Any,
     Iterable,
+    Iterator,
     Literal,
     Optional,
     Union,
@@ -289,6 +290,9 @@ def __init__(
 class DataFrame:
     """Two dimensional table representation of data.
 
+    DataFrame objects are iterable; iterating over a DataFrame yields
+    :class:`pyarrow.RecordBatch` instances lazily.
+
     See :ref:`user_guide_concepts` in the online documentation for more information.
     """
 
@@ -1098,21 +1102,42 @@ def unnest_columns(self, *columns: str, preserve_nulls: bool = True) -> DataFram
         return DataFrame(self.df.unnest_columns(columns, preserve_nulls=preserve_nulls))
 
     def __arrow_c_stream__(self, requested_schema: object | None = None) -> object:
-        """Export an Arrow PyCapsule Stream.
+        """Export the DataFrame as an Arrow C Stream.
 
-        This will execute and collect the DataFrame. We will attempt to respect the
-        requested schema, but only trivial transformations will be applied such as only
-        returning the fields listed in the requested schema if their data types match
-        those in the DataFrame.
+        The DataFrame is executed using DataFusion's streaming APIs and exposed via
+        Arrow's C Stream interface. Record batches are produced incrementally, so the
+        full result set is never materialized in memory. When ``requested_schema`` is
+        provided, only straightforward projections such as column selection or
+        reordering are applied.
 
         Args:
             requested_schema: Attempt to provide the DataFrame using this schema.
 
         Returns:
-            Arrow PyCapsule object.
+            Arrow PyCapsule object representing an ``ArrowArrayStream``.
         """
+        # ``DataFrame.__arrow_c_stream__`` in the Rust extension leverages
+        # ``execute_stream_partitioned`` under the hood to stream batches while
+        # preserving the original partition order.
         return self.df.__arrow_c_stream__(requested_schema)
 
+    def __iter__(self) -> Iterator[pa.RecordBatch]:
+        """Yield record batches from the DataFrame without materializing results.
+
+        This implementation streams record batches via the Arrow C Stream
+        interface, allowing callers such as :func:`pyarrow.Table.from_batches` to
+        consume results lazily. The DataFrame is executed using DataFusion's
+        partitioned streaming APIs so ``collect`` is never invoked and batch
+        order across partitions is preserved.
+        """
+        from contextlib import closing
+
+        import pyarrow as pa
+
+        reader = pa.RecordBatchReader._import_from_c_capsule(self.__arrow_c_stream__())
+        with closing(reader):
+            yield from reader
+
     def transform(self, func: Callable[..., DataFrame], *args: Any) -> DataFrame:
         """Apply a function to the current DataFrame which returns another DataFrame.
 

python/tests/conftest.py

@@ -17,7 +17,7 @@
 
 import pyarrow as pa
 import pytest
-from datafusion import SessionContext
+from datafusion import DataFrame, SessionContext
 from pyarrow.csv import write_csv
 
 
@@ -49,3 +49,12 @@ def database(ctx, tmp_path):
         delimiter=",",
         schema_infer_max_records=10,
     )
+
+
+@pytest.fixture
+def fail_collect(monkeypatch):
+    def _fail_collect(self, *args, **kwargs):  # pragma: no cover - failure path
+        msg = "collect should not be called"
+        raise AssertionError(msg)
+
+    monkeypatch.setattr(DataFrame, "collect", _fail_collect)

python/tests/test_dataframe.py

@@ -46,6 +46,8 @@
 from datafusion.expr import Window
 from pyarrow.csv import write_csv
 
+pa_cffi = pytest.importorskip("pyarrow.cffi")
+
 MB = 1024 * 1024
 
 
@@ -1582,6 +1584,120 @@ def test_empty_to_arrow_table(df):
     assert set(pyarrow_table.column_names) == {"a", "b", "c"}
 
 
+def test_iter_batches_dataframe(fail_collect):
+    ctx = SessionContext()
+
+    batch1 = pa.record_batch([pa.array([1])], names=["a"])
+    batch2 = pa.record_batch([pa.array([2])], names=["a"])
+    df = ctx.create_dataframe([[batch1], [batch2]])
+
+    expected = [batch1, batch2]
+    for got, exp in zip(df, expected):
+        assert got.equals(exp)
+
+
+def test_arrow_c_stream_to_table_and_reader(fail_collect):
+    ctx = SessionContext()
+
+    # Create a DataFrame with two separate record batches
+    batch1 = pa.record_batch([pa.array([1])], names=["a"])
+    batch2 = pa.record_batch([pa.array([2])], names=["a"])
+    df = ctx.create_dataframe([[batch1], [batch2]])
+
+    table = pa.Table.from_batches(df)
+    batches = table.to_batches()
+
+    assert len(batches) == 2
+    assert batches[0].equals(batch1)
+    assert batches[1].equals(batch2)
+    assert table.schema == df.schema()
+    assert table.column("a").num_chunks == 2
+
+    reader = pa.RecordBatchReader._import_from_c_capsule(df.__arrow_c_stream__())
+    assert isinstance(reader, pa.RecordBatchReader)
+    reader_table = pa.Table.from_batches(reader)
+    expected = pa.Table.from_batches([batch1, batch2])
+    assert reader_table.equals(expected)
+
+
+def test_arrow_c_stream_order():
+    ctx = SessionContext()
+
+    batch1 = pa.record_batch([pa.array([1])], names=["a"])
+    batch2 = pa.record_batch([pa.array([2])], names=["a"])
+
+    df = ctx.create_dataframe([[batch1, batch2]])
+
+    table = pa.Table.from_batches(df)
+    expected = pa.Table.from_batches([batch1, batch2])
+
+    assert table.equals(expected)
+    col = table.column("a")
+    assert col.chunk(0)[0].as_py() == 1
+    assert col.chunk(1)[0].as_py() == 2
+
+
+def test_arrow_c_stream_schema_selection(fail_collect):
+    ctx = SessionContext()
+
+    batch = pa.RecordBatch.from_arrays(
+        [
+            pa.array([1, 2]),
+            pa.array([3, 4]),
+            pa.array([5, 6]),
+        ],
+        names=["a", "b", "c"],
+    )
+    df = ctx.create_dataframe([[batch]])
+
+    requested_schema = pa.schema([("c", pa.int64()), ("a", pa.int64())])
+
+    c_schema = pa_cffi.ffi.new("struct ArrowSchema*")
+    address = int(pa_cffi.ffi.cast("uintptr_t", c_schema))
+    requested_schema._export_to_c(address)
+    capsule_new = ctypes.pythonapi.PyCapsule_New
+    capsule_new.restype = ctypes.py_object
+    capsule_new.argtypes = [ctypes.c_void_p, ctypes.c_char_p, ctypes.c_void_p]
+    schema_capsule = capsule_new(ctypes.c_void_p(address), b"arrow_schema", None)
+
+    reader = pa.RecordBatchReader._import_from_c_capsule(
+        df.__arrow_c_stream__(schema_capsule)
+    )
+
+    assert reader.schema == requested_schema
+
+    batches = list(reader)
+
+    assert len(batches) == 1
+    expected_batch = pa.record_batch(
+        [pa.array([5, 6]), pa.array([1, 2])], names=["c", "a"]
+    )
+    assert batches[0].equals(expected_batch)
+
+
+def test_arrow_c_stream_schema_mismatch(fail_collect):
+    ctx = SessionContext()
+
+    batch = pa.RecordBatch.from_arrays(
+        [pa.array([1, 2]), pa.array([3, 4])], names=["a", "b"]
+    )
+    df = ctx.create_dataframe([[batch]])
+
+    bad_schema = pa.schema([("a", pa.string())])
+
+    c_schema = pa_cffi.ffi.new("struct ArrowSchema*")
+    address = int(pa_cffi.ffi.cast("uintptr_t", c_schema))
+    bad_schema._export_to_c(address)
+
+    capsule_new = ctypes.pythonapi.PyCapsule_New
+    capsule_new.restype = ctypes.py_object
+    capsule_new.argtypes = [ctypes.c_void_p, ctypes.c_char_p, ctypes.c_void_p]
+    bad_capsule = capsule_new(ctypes.c_void_p(address), b"arrow_schema", None)
+
+    with pytest.raises(Exception, match="Fail to merge schema"):
+        df.__arrow_c_stream__(bad_capsule)
+
+
 def test_to_pylist(df):
     # Convert datafusion dataframe to Python list
     pylist = df.to_pylist()
@@ -2666,6 +2782,110 @@ def trigger_interrupt():
     interrupt_thread.join(timeout=1.0)
 
 
+def test_arrow_c_stream_interrupted():
+    """__arrow_c_stream__ responds to ``KeyboardInterrupt`` signals.
+
+    Similar to ``test_collect_interrupted`` this test issues a long running
+    query, but consumes the results via ``__arrow_c_stream__``. It then raises
+    ``KeyboardInterrupt`` in the main thread and verifies that the stream
+    iteration stops promptly with the appropriate exception.
+    """
+
+    ctx = SessionContext()
+
+    batches = []
+    for i in range(10):
+        batch = pa.RecordBatch.from_arrays(
+            [
+                pa.array(list(range(i * 1000, (i + 1) * 1000))),
+                pa.array([f"value_{j}" for j in range(i * 1000, (i + 1) * 1000)]),
+            ],
+            names=["a", "b"],
+        )
+        batches.append(batch)
+
+    ctx.register_record_batches("t1", [batches])
+    ctx.register_record_batches("t2", [batches])
+
+    df = ctx.sql(
+        """
+        WITH t1_expanded AS (
+            SELECT
+                a,
+                b,
+                CAST(a AS DOUBLE) / 1.5 AS c,
+                CAST(a AS DOUBLE) * CAST(a AS DOUBLE) AS d
+            FROM t1
+            CROSS JOIN (SELECT 1 AS dummy FROM t1 LIMIT 5)
+        ),
+        t2_expanded AS (
+            SELECT
+                a,
+                b,
+                CAST(a AS DOUBLE) * 2.5 AS e,
+                CAST(a AS DOUBLE) * CAST(a AS DOUBLE) * CAST(a AS DOUBLE) AS f
+            FROM t2
+            CROSS JOIN (SELECT 1 AS dummy FROM t2 LIMIT 5)
+        )
+        SELECT
+            t1.a, t1.b, t1.c, t1.d,
+            t2.a AS a2, t2.b AS b2, t2.e, t2.f
+        FROM t1_expanded t1
+        JOIN t2_expanded t2 ON t1.a % 100 = t2.a % 100
+        WHERE t1.a > 100 AND t2.a > 100
+        """
+    )
+
+    reader = pa.RecordBatchReader._import_from_c_capsule(df.__arrow_c_stream__())
+
+    interrupted = False
+    interrupt_error = None
+    query_started = threading.Event()
+    max_wait_time = 5.0
+
+    def trigger_interrupt():
+        start_time = time.time()
+        while not query_started.is_set():
+            time.sleep(0.1)
+            if time.time() - start_time > max_wait_time:
+                msg = f"Query did not start within {max_wait_time} seconds"
+                raise RuntimeError(msg)
+
+        thread_id = threading.main_thread().ident
+        if thread_id is None:
+            msg = "Cannot get main thread ID"
+            raise RuntimeError(msg)
+
+        exception = ctypes.py_object(KeyboardInterrupt)
+        res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
+            ctypes.c_long(thread_id), exception
+        )
+        if res != 1:
+            ctypes.pythonapi.PyThreadState_SetAsyncExc(
+                ctypes.c_long(thread_id), ctypes.py_object(0)
+            )
+            msg = "Failed to raise KeyboardInterrupt in main thread"
+            raise RuntimeError(msg)
+
+    interrupt_thread = threading.Thread(target=trigger_interrupt)
+    interrupt_thread.daemon = True
+    interrupt_thread.start()
+
+    try:
+        query_started.set()
+        # consume the reader which should block and be interrupted
+        reader.read_all()
+    except KeyboardInterrupt:
+        interrupted = True
+    except Exception as e:  # pragma: no cover - unexpected errors
+        interrupt_error = e
+
+    if not interrupted:
+        pytest.fail(f"Stream was not interrupted; got error: {interrupt_error}")
+
+    interrupt_thread.join(timeout=1.0)
+
+
 def test_show_select_where_no_rows(capsys) -> None:
     ctx = SessionContext()
     df = ctx.sql("SELECT 1 WHERE 1=0")