You could do this fairly efficiently by leveraging the CountingIterator
to get an exact count (taking visibilities into account, as well) for
the range in question. It isn't going to be as fast as a precomputed
answer, but you could cache that easily.
The fact that visibilities will affect the cardinality of a term makes
it harder for us to provide this within Accumulo. The situations where
Accumulo itself cares about cardinality, it's agnostic of the
visibilities. It would be possible to try to build an index of this
information internally, but, like Eric said, that's not there today.
On 6/27/14, 10:40 AM, Eric Newton wrote:
Short answer: no.
Long answer:
You can scan the metadata table for the count/size of the files.
You can query tablet servers for the basic stats of every tablet for a
given table. This is used for balancing.
But really you should collect the statistics you want during ingest and
insert them in another table.
-Eric
On Fri, Jun 27, 2014 at 9:42 AM, Jamie Stephens <[email protected]
<mailto:[email protected]>> wrote:
Is there a way to get a quick estimate of the number of keys in a
given range?
Perhaps more generally, getting an estimate of the amount of work
(and even some sort of confidence based on, say, the age of
something) to iterate over a range.
I'd like to do some query planning, so statistics like these sure
would be nice.
--Jamie
