initial draft of reduce function doc

content/v2.0/reference/flux/functions/built-in/transformations/reduce.md

@@ -0,0 +1,117 @@
+---
+title: reduce() function
+description: The `reduce()` function aggregates records in each table according to the reducer, `fn`.
+menu:
+  v2_0_ref:
+    name: reduce
+    parent: built-in-transformations
+weight: 401
+---
+
+The `reduce()` function aggregates records in each table according to the reducer, `fn`.
+The output for each table is the group key of the table with columns corresponding
+to each field in the reducer object.
+
+If the reducer record contains a column with the same name as a group key column,
+the group key column's value is overwritten and the resulting record is regrouped
+into the appropriate table.
+
+_**Function type:** Transformation_
+
+```js
+reduce(
+  fn: (r, accumulator) => ({ sum: r._value + accumulator.sum }),
+  identity: {sum: 0.0}
+)
+```
+
+## Parameters
+
+### fn
+Function to apply to each record with a reducer object ([`identity`](#identity)).
+
+_**Data type:** Function_
+
+###### fn syntax
+```js
+// Pattern
+fn: (r, accumulator) => ({ newColumnName: r.column + accumulator.identityKey })
+
+// Example
+fn: (r, accumulator) => ({ valueSum: r._value + accumulator.sum })
+```
+
+#### r
+Object representing each row or record.
+
+#### accumulator
+Reducer object defined by [`identity`](#identity).
+
+### identity
+Defines the reducer object and provides initial values to use when creating a reducer.
+May be used more than once in asynchronous processing use cases.
+_The data type of values in the `identity` object determine the data type of output values._
+
+_**Data type:** Object_
+
+###### identity object syntax
+```js
+// Pattern
+identity: {key1: value1, key2: value2}
+
+// Example
+identity: {sum: 0.0, count: 0.0}
+```
+
+## Examples
+
+##### Compute the sum of the value column
+```js
+from(bucket:"example-bucket")
+    |> filter(fn: (r) =>
+        r._measurement == "cpu" and
+        r._field == "usage_system" and
+        r.service == "app-server"
+    )
+    |> range(start:-12h)
+    |> reduce(
+        fn: (r, accumulator) => ({
+            sum: r._value + accumulator.sum
+        }),
+        identity: {sum: 0.0}
+    )
+```
+
+##### Compute the sum and count in a single reducer
+```js
+from(bucket:"example-bucket")
+    |> filter(fn: (r) =>
+        r._measurement == "cpu" and
+        r._field == "usage_system" and
+        r.service == "app-server"
+    )
+    |> range(start:-12h)
+    |> reduce(
+        fn: (r, accumulator) => ({
+          valueSum: r._value + accumulator.sum,
+          valueCount: accumulator.count + 1.0
+        }),
+        identity: {sum: 0.0, count: 0.0}
+    )
+```
+
+##### Compute the product of all values
+```js
+from(bucket:"example-bucket")
+    |> filter(fn: (r) =>
+        r._measurement == "cpu" and
+        r._field == "usage_system" and
+        r.service == "app-server")
+    |> range(start:-12h)
+    |> reduce(
+        fn: (r, accumulator) => ({
+            valueProduct: r._value * accumulator.prod
+        }),
+        identity: {prod: 1.0}        
+    )
+```