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From Russ Weeks <rwe...@newbrightidea.com>
Subject Calculating averages with eg. StatsCombiner
Date Fri, 11 Jul 2014 07:38:24 GMT
Hi,

I'd like to understand this paragraph in the Accumulo manual a little
better:

"The only restriction on an combining iterator is that the combiner
developer should not assume that all values for a given key have been seen,
since new mutations can be inserted at anytime. This precludes using the
total number of values in the aggregation such as when calculating an
average, for example."

By "using the total number of values in the aggregation", I presume that it
means inside the combiner's reduce method? Because it seems like if I'm
using the example StatsCombiner registered on all 3 scopes, after the scan
completes the count and the sum fields should be consistent (w.r.t each
other, of course new mutations could have been added since the scan
started) and if I divide the two I'll get an accurate average, right?

Thanks,
-Russ

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