I am not sure why second option is better - in both cases, you are
shipping only the combined counts from map to reduce.
On other hand, first could be better since it means we need to project
only 'a1' - and none of the other fields.
Or did I miss something here ?
I am not very familiar to what pig does in this case right now.
Regards,
Mridul
On Thursday 26 August 2010 03:45 AM, Dmitriy Ryaboy wrote:
Generally speaking, the second option will be more performant as it might
let you drop column a3 early. In most cases the magnitude of this is likely
to be very small as COUNT is an algebraic function, so most of the work is
done map-side anyway, and only partial, pre-aggregated counts are shipped
from mappers to reducers. However, if A is very wide, or a column store, or
has non-negligible deserialization cost that can be offset by only
deserializing a few fields -- the second option is better.
-D
On Wed, Aug 25, 2010 at 1:58 PM, Corbin Hoenes<cor...@tynt.com> wrote:
Wondering about performance and count...
A = load 'test.csv' as (a1,a2,a3);
B = GROUP A by a1;
-- which preferred?
C = FOREACH B GENERATE COUNT(A);
-- or would this only send a single field through the COUNT and be more
performant?
C = FOREACH B GENERATE COUNT(A.a2);
