[GitHub] [arrow] westonpace commented on a diff in pull request #35320: GH-32335: [C++][Docs] Add design document for Acero

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on code in PR #35320:
URL: https://github.com/apache/arrow/pull/35320#discussion_r1192738776


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docs/source/cpp/acero/substrait.rst:
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@@ -0,0 +1,248 @@
+.. Licensed to the Apache Software Foundation (ASF) under one
+.. or more contributor license agreements.  See the NOTICE file
+.. distributed with this work for additional information
+.. regarding copyright ownership.  The ASF licenses this file
+.. to you under the Apache License, Version 2.0 (the
+.. "License"); you may not use this file except in compliance
+.. with the License.  You may obtain a copy of the License at
+
+..   http://www.apache.org/licenses/LICENSE-2.0
+
+.. Unless required by applicable law or agreed to in writing,
+.. software distributed under the License is distributed on an
+.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+.. KIND, either express or implied.  See the License for the
+.. specific language governing permissions and limitations
+.. under the License.
+
+.. default-domain:: cpp
+.. highlight:: cpp
+.. cpp:namespace:: arrow::engine::substrait
+
+.. _acero-substrait:
+
+==========================
+Using Acero with Substrait
+==========================
+
+In order to use Acero you will need to create an execution plan.  This is the
+model that describes the computation you want to apply to your data.  Acero has
+its own internal representation for execution plans but most users should not
+interact with this directly as it will couple their code to Acero.
+
+`Substrait <https://substrait.io>`_ is an open standard for execution plans.
+Acero implements the Substrait "consumer" interface.  This means that Acero can
+accept a Substrait plan and fulfill the plan, loading the requested data and
+applying the desired computation.  By using Substrait plans users can easily
+switch out to a different execution engine at a later time.
+
+Substrait Conformance
+---------------------
+
+Substrait defines a broad set of operators and functions for many different
+situations and it is unlikely that Acero will ever completely satisfy all
+defined Substrait operators and functions.  To help understand what features
+are available the following sections define which features have been currently
+implemented in Acero and any caveats that apply.
+
+Plans
+^^^^^
+
+ * A plan should have a single top-level relation.
+ * The consumer is currently based on version 0.20.0 of Substrait.
+   Any features added that are newer will not be supported.
+ * Due to a breaking change in 0.20.0 any Substrait plan older than 0.20.0
+   will be rejected.
+
+Extensions
+^^^^^^^^^^
+
+ * If a plan contains any extension type variations it will be rejected.
+ * Advanced extensions can be provided by supplying a custom implementation of
+   :class:`arrow::engine::ExtensionProvider`.
+
+Relations (in general)
+^^^^^^^^^^^^^^^^^^^^^^
+
+ * Any relation not explicitly listed below will not be supported
+   and will cause the plan to be rejected.
+
+Read Relations
+^^^^^^^^^^^^^^
+
+ * The ``projection`` property is not supported and plans containing this
+   property will be rejected.
+ * The ``VirtualTable`` and ``ExtensionTable`` read types are not supported.
+   Plans containing these types will be rejected.
+ * Only the parquet and arrow file formats are currently supported.
+ * All URIs must use the ``file`` scheme
+ * ``partition_index``, ``start``, and ``length`` are not supported.  Plans containing
+   non-default values for these properties will be rejected.
+ * The Substrait spec requires that a ``filter`` be completely satisfied by a read
+   relation.  However, Acero only uses a read filter for pushdown projection and
+   it may not be fully satisfied.  Users should generally attach an additional
+   filter relation with the same filter expression after the read relation.
+
+Filter Relations
+^^^^^^^^^^^^^^^^
+
+ * No known caveats
+
+Project Relations
+^^^^^^^^^^^^^^^^^
+
+ * No known caveats
+
+Join Relations
+^^^^^^^^^^^^^^
+
+ * The join type ``JOIN_TYPE_SINGLE`` is not supported and plans containing this
+   will be rejected.
+ * The join expression must be a call to either the ``equal`` or ``is_not_distinct_from``
+   functions.  Both arguments to the call must be direct references.  Only a single
+   join key is supported.
+ * The ``post_join_filter`` property is not supported and will be ignored.
+
+Aggregate Relations
+^^^^^^^^^^^^^^^^^^^
+
+ * At most one grouping set is supported.
+ * Each grouping expression must be a direct reference.
+ * Each measure's arguments must be direct references.
+ * A measure may not have a filter
+ * A measure may not have sorts
+ * A measure's invocation must be AGGREGATION_INVOCATION_ALL or 
+   AGGREGATION_INVOCATION_UNSPECIFIED
+ * A measure's phase must be AGGREGATION_PHASE_INITIAL_TO_RESULT
+
+Expressions (general)
+^^^^^^^^^^^^^^^^^^^^^
+
+ * Various places in the Substrait spec allow for expressions to be used outside
+   of a filter or project relation.  For example, a join expression or an aggregate
+   grouping set.  Acero typically expects these expressions to be direct references.
+   Planners should extract the implicit projection into a formal project relation
+   before delivering the plan to Acero.
+
+Literals
+^^^^^^^^
+
+ * A literal with non-default nullability will cause a plan to be rejected.
+
+Types
+^^^^^
+
+ * Acero does not have full support for non-nullable types and may allow input
+   to have nulls without rejecting it.
+ * The table below shows the mapping between Arrow types and Substrait type
+   classes that are currently supported
+
+.. list-table:: Substrait / Arrow Type Mapping
+   :widths: 25 25 50
+   :header-rows: 1
+
+   * - Substrait Type
+     - Arrow Type
+     - Caveat
+   * - boolean
+     - boolean
+     - 
+   * - i8
+     - int8
+     - 
+   * - i16
+     - int16
+     - 
+   * - i32
+     - int32
+     - 
+   * - i64
+     - int64
+     - 
+   * - fp32
+     - float32
+     - 
+   * - fp64
+     - float64
+     - 
+   * - string
+     - string
+     - 
+   * - binary
+     - binary
+     - 
+   * - timestamp
+     - timestamp<MICRO,"">
+     - 
+   * - timestamp_tz
+     - timestamp<MICRO,"UTC">
+     - 
+   * - date
+     - date32<DAY>
+     - 
+   * - time
+     - time64<MICRO>
+     - 
+   * - interval_year
+     - 
+     - Not currently supported
+   * - interval_day
+     - 
+     - Not currently supported
+   * - uuid
+     - 
+     - Not currently supported
+   * - FIXEDCHAR<L>
+     - 
+     - Not currently supported
+   * - VARCHAR<L>
+     - 
+     - Not currently supported
+   * - FIXEDBINARY<L>
+     - fixed_size_binary<L>
+     - 
+   * - DECIMAL<P,S>
+     - decimal128<P,S>
+     - 
+   * - STRUCT<T1...TN>
+     - struct<T1...TN>
+     - Arrow struct fields will have no name (empty string)
+   * - NSTRUCT<N:T1...N:Tn>
+     - 
+     - Not currently supported
+   * - LIST<T>
+     - list<T>
+     - 
+   * - MAP<K,V>
+     - map<K,V>
+     - K must not be nullable
+
+Functions
+^^^^^^^^^
+
+ * The following functions have caveats or are not supported at all.  Note that
+   this is not a comprehensive list.  Functions are being added to Substrait at
+   a rapid pace and new functions may be missing.
+
+   * Acero does not support the SATURATE option for overflow
+   * Acero does not support kernels that take more than two arguments
+     for the functions ``and``, ``or``, ``xor``
+   * Acero does not support temporal arithmetic

Review Comment:
   I think that's a fair point.  Let's address in a follow-up.



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