Fabian Hueske created FLINK-1656:
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Summary: Fix ForwardedField documentation for operators with
iterator input
Key: FLINK-1656
URL: https://issues.apache.org/jira/browse/FLINK-1656
Project: Flink
Issue Type: Bug
Components: Documentation
Affects Versions: 0.9
Reporter: Fabian Hueske
Assignee: Fabian Hueske
Priority: Critical
The documentation of ForwardedFields is incomplete for operators with iterator
inputs (GroupReduce, CoGroup).
This should be fixed ASAP, because it can lead to incorrect program execution.
The conditions for forwarded fields on operators with iterator input are:
1) forwarded fields must be emitted in the order in which they are received
through the iterator
2) all forwarded fields of a record must stick together, i.e., if your function
builds record from field 0 of the 1st, 3rd, 5th, ... and field 1 of the 2nd,
4th, ... record coming through the iterator, these are not valid forwarded
fields.
3) it is OK to completely filter out records coming through the iterator.
The reason for these conditions is that the optimizer uses forwarded fields to
reason about physical data properties such as order and grouping. Mixing up the
order of records or emitting records which are composed from different input
records, might destroy a (secondary) order or grouping.
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