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]
Zhong Yanghong commented on KYLIN3361:

The sql
{code}
select A, stddev(SUM_M)
from (
select A, sampling_dim, sum(m) as SUM_M
from T
group by A, sampling_dim
) a
group by A
{code}
now can be expressed with udf *stddev_sum* like this
{code}
select A, stddev_sum(sampling_dim, m)
from T
group by A
{code}
> Support stddev as a new measure
> 
>
> Key: KYLIN3361
> URL: https://issues.apache.org/jira/browse/KYLIN3361
> Project: Kylin
> Issue Type: Subtask
> Reporter: Zhong Yanghong
> Assignee: Zhong Yanghong
> Priority: Major
> Fix For: Backlog
>
>
> (x ~1~  +x+) ^2^ + (x ~2~  +x+) ^2^ + ... + (x ~n~  +x+) ^2^ = x ~1~ ^2^ + x ~2~ ^2^
+ ... + x ~n~ ^2^  n +x+ ^2^, where +x+ is the average of x ~1~, x ~2~, ..., x ~n~. Therefore,
to compute stddev, what kylin need to do is to precalculate sum(x ~i~ ^2^), sum(x ~i~) and
count

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