[jira] Commented: (HIVE-165) var(col) built-in to go with avg(col) and count(col)

["David Phillips (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-165?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12656219#action_12656219 ] 

David Phillips commented on HIVE-165:
-------------------------------------

Rather than add a non-standard VAR function, we should add these ANSI SQL 2003 functions:

COVAR_POP
COVAR_SAMP
STDDEV_POP
STDDEV_SAMP
VAR_POP
VAR_SAMP

I recommend against adding the non-standard STDDEV and VARIANCE functions as their behavior differs between MySQL (population) and Oracle/PostgreSQL (sample).

For the time being, you can use this workaround:

VAR_POP = (SUM(x * x) - (SUM(x) * AVG(x))) / COUNT(x)
VAR_SAMP = (SUM(x * x) - (SUM(x) * AVG(x))) / (COUNT(x) - 1)



> var(col) built-in to go with avg(col) and count(col)
> ----------------------------------------------------
>
>                 Key: HIVE-165
>                 URL: https://issues.apache.org/jira/browse/HIVE-165
>             Project: Hadoop Hive
>          Issue Type: Wish
>            Reporter: Adam Kramer
>            Priority: Minor
>
> The last step in the unholy triumvirate of statistical built-ins is the variance. We already have the n (count) and the mean (avg). I currently have a job or two that filters all of the data into a single reducer which just computes mean/n/variance and writes it to a table...so my guess is that this would be a pretty big speed increase. Not a huge deal though, as computing the variance myself is trivial.
> (Average, variance, and n can be co-computed in one pass, so if you're doing var() you can basically have avg() and count() for free.)

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