[FLINK-1521] Chained operators respect reuse

[zentol]

No description provided.

[StephanEwen]

While this change makes it technically more correct, I think it will have a massive negative impact on performance. Nobody can be expected to turn on the object reuse switch without being a seriously advanced user. But everybody will experience the performance drop.

Since no user ever ran into this problem, I think we should NOT change it at this point.

[fhueske]

I agree with Stephan to not copy records for chained mappers.
However, we should clearly document the behavior of the different modes or at least the behavior of the default mode.

[gyfora]

The problem here I think that an error caused by the reusing mapper could be very hard to detect. So some users might have it but they don't realise.

For streaming we copy the input at every chained function call just to be on the safe side, but we'll have to reintroduce the object reuse mode.

[StephanEwen]

At this point, we also have to worry about efficiency and performance.
This is also something that the user can easily mitigate in the UDF, at a fraction of the cost in most cases.

I vote postpone the fix until we get some tangible user feedback concerning this issue.

[fhueske]

+1
I opened FLINK-1565 for documenting object reuse behavior.

[zentol]

Alright, I`m closing this PR then.