If you don't specify grouping comparator for your Job, it uses the Output Key Comparator class for grouping.
This comparator should be provided if the equivalence rules for keys sorting the intermediates are different from those for grouping keys.
From: Ajay Srivastava [[EMAIL PROTECTED]]
Sent: Monday, June 04, 2012 4:26 PM
To: [EMAIL PROTECTED]
Subject: Grouping comparator
Can someone please explain default implementation of grouping comparator i.e. if I do not specify a custom grouping comparator then which comparator is called to decide the grouping for reducer.
I searched a lot on web but could not find a satisfactory explanation for its default implementation.