docs/source/user-guide/data-sources.rst

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.. _user_guide_data_sources:

Data Sources
============

DataFusion provides a wide variety of ways to get data into a DataFrame to perform operations.

Local file
----------

DataFusion has the ability to read from a variety of popular file formats, such as :ref:`Parquet <io_parquet>`,
:ref:`CSV <io_csv>`, :ref:`JSON <io_json>`, and :ref:`AVRO <io_avro>`.

.. ipython:: python

    from datafusion import SessionContext
    ctx = SessionContext()
    df = ctx.read_csv("pokemon.csv")
    df.show()

Create in-memory
----------------

Sometimes it can be convenient to create a small DataFrame from a Python list or dictionary object.
To do this in DataFusion, you can use one of the three functions
:py:func:`~datafusion.context.SessionContext.from_pydict`,
:py:func:`~datafusion.context.SessionContext.from_pylist`, or
:py:func:`~datafusion.context.SessionContext.create_dataframe`.

As their names suggest, ``from_pydict`` and ``from_pylist`` will create DataFrames from Python
dictionary and list objects, respectively. ``create_dataframe`` assumes you will pass in a list
of list of `PyArrow Record Batches <https://arrow.apache.org/docs/python/generated/pyarrow.RecordBatch.html>`_.

The following three examples all will create identical DataFrames:

.. ipython:: python

    import pyarrow as pa

    ctx.from_pylist([
        { "a": 1, "b": 10.0, "c": "alpha" },
        { "a": 2, "b": 20.0, "c": "beta" },
        { "a": 3, "b": 30.0, "c": "gamma" },
    ]).show()

    ctx.from_pydict({
        "a": [1, 2, 3],
        "b": [10.0, 20.0, 30.0],
        "c": ["alpha", "beta", "gamma"],
    }).show()

    batch = pa.RecordBatch.from_arrays(
        [
            pa.array([1, 2, 3]),
            pa.array([10.0, 20.0, 30.0]),
            pa.array(["alpha", "beta", "gamma"]),
        ],
        names=["a", "b", "c"],
    )

    ctx.create_dataframe([[batch]]).show()


Object Store
------------

DataFusion has support for multiple storage options in addition to local files.
The example below requires an appropriate S3 account with access credentials.

Supported Object Stores are

- :py:class:`~datafusion.object_store.AmazonS3`
- :py:class:`~datafusion.object_store.GoogleCloud`
- :py:class:`~datafusion.object_store.Http`
- :py:class:`~datafusion.object_store.LocalFileSystem`
- :py:class:`~datafusion.object_store.MicrosoftAzure`

.. code-block:: python

    from datafusion.object_store import AmazonS3

    region = "us-east-1"
    bucket_name = "yellow-trips"

    s3 = AmazonS3(
        bucket_name=bucket_name,
        region=region,
        access_key_id=os.getenv("AWS_ACCESS_KEY_ID"),
        secret_access_key=os.getenv("AWS_SECRET_ACCESS_KEY"),
    )

    path = f"s3://{bucket_name}/"
    ctx.register_object_store("s3://", s3, None)

    ctx.register_parquet("trips", path)

    ctx.table("trips").show()

Other DataFrame Libraries
-------------------------

DataFusion can import DataFrames directly from other libraries, such as
`Polars <https://pola.rs/>`_ and `Pandas <https://pandas.pydata.org/>`_.
Since DataFusion version 42.0.0, any DataFrame library that supports the Arrow FFI PyCapsule
interface can be imported to DataFusion using the
:py:func:`~datafusion.context.SessionContext.from_arrow` function. Older versions of Polars may
not support the arrow interface. In those cases, you can still import via the
:py:func:`~datafusion.context.SessionContext.from_polars` function.

.. code-block:: python

    import pandas as pd

    data = { "a": [1, 2, 3], "b": [10.0, 20.0, 30.0], "c": ["alpha", "beta", "gamma"] }
    pandas_df = pd.DataFrame(data)

    datafusion_df = ctx.from_arrow(pandas_df)
    datafusion_df.show()

.. code-block:: python

    import polars as pl
    polars_df = pl.DataFrame(data)

    datafusion_df = ctx.from_arrow(polars_df)
    datafusion_df.show()

Delta Lake
----------

DataFusion 43.0.0 and later support the ability to register table providers from sources such
as Delta Lake. This will require a recent version of
`deltalake <https://delta-io.github.io/delta-rs/>`_ to provide the required interfaces.

.. code-block:: python

    from deltalake import DeltaTable

    delta_table = DeltaTable("path_to_table")
    ctx.register_table("my_delta_table", delta_table)
    df = ctx.table("my_delta_table")
    df.show()

On older versions of ``deltalake`` (prior to 0.22) you can use the
`Arrow DataSet <https://arrow.apache.org/docs/python/generated/pyarrow.dataset.Dataset.html>`_
interface to import to DataFusion, but this does not support features such as filter push down
which can lead to a significant performance difference.

.. code-block:: python

    from deltalake import DeltaTable

    delta_table = DeltaTable("path_to_table")
    ctx.register_dataset("my_delta_table", delta_table.to_pyarrow_dataset())
    df = ctx.table("my_delta_table")
    df.show()

Apache Iceberg
--------------

DataFusion 45.0.0 and later support the ability to register Apache Iceberg tables as table providers through the Custom Table Provider interface.

This requires either the `pyiceberg <https://pypi.org/project/pyiceberg/>`__ library (>=0.10.0) or the `pyiceberg-core <https://pypi.org/project/pyiceberg-core/>`__ library (>=0.5.0).

* The ``pyiceberg-core`` library exposes Iceberg Rust's implementation of the Custom Table Provider interface as python bindings.
* The ``pyiceberg`` library utilizes the ``pyiceberg-core`` python bindings under the hood and provides a native way for Python users to interact with the DataFusion.

.. code-block:: python

    from datafusion import SessionContext
    from pyiceberg.catalog import load_catalog
    import pyarrow as pa

    # Load catalog and create/load a table
    catalog = load_catalog("catalog", type="in-memory")
    catalog.create_namespace_if_not_exists("default")

    # Create some sample data
    data = pa.table({"x": [1, 2, 3], "y": [4, 5, 6]})
    iceberg_table = catalog.create_table("default.test", schema=data.schema)
    iceberg_table.append(data)

    # Register the table with DataFusion
    ctx = SessionContext()
    ctx.register_table_provider("test", iceberg_table)

    # Query the table using DataFusion
    ctx.table("test").show()


Note that the Datafusion integration rely on features from the `Iceberg Rust <https://github.com/apache/iceberg-rust/>`_ implementation instead of the `PyIceberg <https://github.com/apache/iceberg-python/>`_ implementation. 
Features that are available in PyIceberg but not yet in Iceberg Rust will not be available when using DataFusion.

Custom Table Provider
---------------------

You can implement a custom Data Provider in Rust and expose it to DataFusion through the
the interface as describe in the :ref:`Custom Table Provider <io_custom_table_provider>`
section. This is an advanced topic, but a
`user example <https://github.com/apache/datafusion-python/tree/main/examples/datafusion-ffi-example>`_
is provided in the DataFusion repository.

Catalog
=======

A common technique for organizing tables is using a three level hierarchical approach. DataFusion
supports this form of organizing using the :py:class:`~datafusion.catalog.Catalog`,
:py:class:`~datafusion.catalog.Schema`, and :py:class:`~datafusion.catalog.Table`. By default,
a :py:class:`~datafusion.context.SessionContext` comes with a single Catalog and a single Schema
with the names ``datafusion`` and ``default``, respectively.

The default implementation uses an in-memory approach to the catalog and schema. We have support
for adding additional in-memory catalogs and schemas. This can be done like in the following
example:

.. code-block:: python

    from datafusion.catalog import Catalog, Schema

    my_catalog = Catalog.memory_catalog()
    my_schema = Schema.memory_schema()

    my_catalog.register_schema("my_schema_name", my_schema)

    ctx.register_catalog("my_catalog_name", my_catalog)

You could then register tables in ``my_schema`` and access them either through the DataFrame
API or via sql commands such as ``"SELECT * from my_catalog_name.my_schema_name.my_table"``.

User Defined Catalog and Schema
-------------------------------

If the in-memory catalogs are insufficient for your uses, there are two approaches you can take
to implementing a custom catalog and/or schema. In the below discussion, we describe how to
implement these for a Catalog, but the approach to implementing for a Schema is nearly
identical.

DataFusion supports Catalogs written in either Rust or Python. If you write a Catalog in Rust,
you will need to export it as a Python library via PyO3. There is a complete example of a
catalog implemented this way in the
`examples folder <https://github.com/apache/datafusion-python/tree/main/examples/>`_
of our repository. Writing catalog providers in Rust provides typically can lead to significant
performance improvements over the Python based approach.

To implement a Catalog in Python, you will need to inherit from the abstract base class
:py:class:`~datafusion.catalog.CatalogProvider`. There are examples in the
`unit tests <https://github.com/apache/datafusion-python/tree/main/python/tests>`_ of
implementing a basic Catalog in Python where we simply keep a dictionary of the
registered Schemas.

One important note for developers is that when we have a Catalog defined in Python, we have
two different ways of accessing this Catalog. First, we register the catalog with a Rust
wrapper. This allows for any rust based code to call the Python functions as necessary.
Second, if the user access the Catalog via the Python API, we identify this and return back
the original Python object that implements the Catalog. This is an important distinction
for developers because we do *not* return a Python wrapper around the Rust wrapper of the
original Python object.