import quincy 17.2.0

[ceph.git] / ceph / src / arrow / cpp / src / arrow / compute / README.md

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+<!---
+  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.
+-->
+
+## Apache Arrow C++ Compute Functions
+
+This submodule contains analytical functions that process primarily Arrow
+columnar data; some functions can process scalar or Arrow-based array
+inputs. These are intended for use inside query engines, data frame libraries,
+etc.
+
+Many functions have SQL-like semantics in that they perform elementwise or
+scalar operations on whole arrays at a time. Other functions are not SQL-like
+and compute results that may be a different length or whose results depend on
+the order of the values.
+
+Some basic terminology:
+
+* We use the term "function" to refer to particular general operation that may
+  have many different implementations corresponding to different combinations
+  of types or function behavior options.
+* We call a specific implementation of a function a "kernel". When executing a
+  function on inputs, we must first select a suitable kernel (kernel selection
+  is called "dispatching") corresponding to the value types of the inputs
+* Functions along with their kernel implementations are collected in a
+  "function registry". Given a function name and argument types, we can look up
+  that function and dispatch to a compatible kernel.
+
+Types of functions
+
+* Scalar functions: elementwise functions that perform scalar operations in a
+  vectorized manner. These functions are generally valid for SQL-like
+  context. These are called "scalar" in that the functions executed consider
+  each value in an array independently, and the output array or arrays have the
+  same length as the input arrays. The result for each array cell is generally
+  independent of its position in the array.
+* Vector functions, which produce a result whose output is generally dependent
+  on the entire contents of the input arrays. These functions **are generally
+  not valid** for SQL-like processing because the output size may be different
+  than the input size, and the result may change based on the order of the
+  values in the array. This includes things like array subselection, sorting,
+  hashing, and more.
+* Scalar aggregate functions of which can be used in a SQL-like context
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