Why is using Combiner function not preferred in shuffling?

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Though a combiner function reduces the shuffling, it is not a preferred method and merging keys are used instead. My question is, why is it not preferred?
Jan 22 in Big Data Hadoop by slayer
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Combiner function is not preferred because when a Combiner function is used, there is no way of controlling how many times and if at all Combiner will actually be used.
The default Shuffle & Sort mechanism is based on alphabetical sorting and hash shuffling of the keys and is preferred over Combiner function.

answered Jan 22 by Omkar
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