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From Steven Wong <sw...@netflix.com>
Subject RE: Last value for a column
Date Sat, 28 Jan 2012 04:28:26 GMT
Other than writing a custom UDAF or TRANSFORM script, a somewhat ugly way is something like:

SELECT user_id, split(max(concat(time, '_', colour)), '_')[1]
FROM T
GROUP BY user_id


From: mdefoinplatel.ext@orange.com [mailto:mdefoinplatel.ext@orange.com]
Sent: Thursday, January 26, 2012 3:24 AM
To: user@hive.apache.org
Subject: Last value for a column

Dear all,
I am struggling with a Hive query where I am trying to get the last value for a column.

Let say I have a table T with three columns: user_id, time, colour and I want to know for
each user_id what is its last colour value.

At the moment I am using the following (naïve) query:

SELECT T1.user_id, T1.lastTime, T2.colour FROM (
              SELECT user_id, max(Time) AS lastTime FROM T GROUP BY user_id ) T1
             JOIN T T2 ON (T1.user_id=T2.user_id AND T2.time=T1.lastTime)

I am not happy with this query of course since I have to read the table T twice.

So I have three questions:

1)      Is there a way to re-write this query in a more efficient way ?



2)      Wouldn't that be better to  stream the output of a distributed by on user_id into
a python script and print out only one row per user_id  (assuming  sorted by user_id and time)
?





3)      Couldn't I create the table T with cluster by on user_id and sorted by time at the
first place and then use this structure to extract the rows I am interested in ?


Many thanks for your help,
Michael

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