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From "Prateek Rungta (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-11742) last_value window specifier enforces ordering as a partition
Date Fri, 04 Sep 2015 21:55:45 GMT
Prateek Rungta created HIVE-11742:
-------------------------------------

             Summary: last_value window specifier enforces ordering as a partition
                 Key: HIVE-11742
                 URL: https://issues.apache.org/jira/browse/HIVE-11742
             Project: Hive
          Issue Type: Bug
          Components: PTF-Windowing
            Reporter: Prateek Rungta


[HIVE-4262|https://issues.apache.org/jira/browse/HIVE-4262] changed the partitioning behavior
of the last_value function. For a specified last_value() OVER X. The ordering spec within
X is used in addition to the partition spec for partitioning. i.e. last_value(a) OVER (PARTITION
BY i ORDER BY j) operates last_value(a) on all rows within the unique combination of (i,j).
The behavior I'd expect is for PARTITION BY i to define the partitioning, and ORDER BY to
define the ordering within the PARTITION. i.e. last_value(a) OVER (PARTITION BY i ORDER BY
j) should operate last_value(a) on all rows within the unique values of (i), ordered by j
within the partition. 

This was changed to be consistent with how SQLServer handled such queries. [SQLServer Docs|https://msdn.microsoft.com/en-us/library/hh231517.aspx?f=255&MSPPError=-2147217396]
describe their example (which performs as Hive does): 

{quote}
The PARTITION BY clause partitions the employees by department and the LAST_VALUE function
is applied to each partition independently. The ORDER BY clause specified in the OVER clause
determines the logical order in which the LAST_VALUE function is applied to the rows in each
partition.
{quote}

To me, their behavior is inconsistent with their description. I've filled an [upstream bug|https://connect.microsoft.com/SQLServer/feedback/details/1753482]
with Microsoft for the same. 

[Oracle|https://oracle-base.com/articles/misc/first-value-and-last-value-analytic-functions]
and [Redshift|http://docs.aws.amazon.com/redshift/latest/dg/r_Examples_of_firstlast_WF.html]
both exhibit the behavior I'd expect.

Considering Hive-4262 has been in core for 2+ years, I don't think we can change the behavior
without potentially impacting clients. But I would like a way to enable the expected behavior
at the least (behind a config flag maybe?). What do you think?



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