[2/2] flink git commit: [FLINK-2883] [docs] Add documentation to forbid key-modifying ReduceFunction

[gr...@apache.org]

[FLINK-2883] [docs] Add documentation to forbid key-modifying ReduceFunction

This closes #3256


Project: http://git-wip-us.apache.org/repos/asf/flink/repo
Commit: http://git-wip-us.apache.org/repos/asf/flink/commit/11ebf484
Tree: http://git-wip-us.apache.org/repos/asf/flink/tree/11ebf484
Diff: http://git-wip-us.apache.org/repos/asf/flink/diff/11ebf484

Branch: refs/heads/master
Commit: 11ebf484280314231d146dcfb0b973934448f00b
Parents: 327ff06
Author: Greg Hogan <co...@greghogan.com>
Authored: Thu Feb 2 16:15:52 2017 -0500
Committer: Greg Hogan <co...@greghogan.com>
Committed: Mon Feb 6 14:40:10 2017 -0500

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 docs/dev/batch/dataset_transformations.md | 4 ++++
 1 file changed, 4 insertions(+)
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http://git-wip-us.apache.org/repos/asf/flink/blob/11ebf484/docs/dev/batch/dataset_transformations.md
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diff --git a/docs/dev/batch/dataset_transformations.md b/docs/dev/batch/dataset_transformations.md
index 1bfb87c..1f942d4 100644
--- a/docs/dev/batch/dataset_transformations.md
+++ b/docs/dev/batch/dataset_transformations.md
@@ -279,6 +279,10 @@ element using a user-defined reduce function.
 For each group of input elements, a reduce function successively combines pairs of elements into one
 element until only a single element for each group remains.
 
+Note that for a `ReduceFunction` the keyed fields of the returned object should match the input
+values. This is because reduce is implicitly combinable and objects emitted from the combine
+operator are again grouped by key when passed to the reduce operator.
+
 #### Reduce on DataSet Grouped by Key Expression
 
 Key expressions specify one or more fields of each element of a DataSet. Each key expression is