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From David Kulp <dk...@fiksu.com>
Subject Re: Lag function in Hive
Date Tue, 10 Apr 2012 14:45:25 GMT
New here.  Hello all.

Could you try a self-join, possibly also restricted to partitions?

E.g. SELECT t2.value - t1.value FROM mytable t1, mytable t2 WHERE t1.rownum = t2.rownum+1
AND t1.partition=foo AND t2.partition=bar

If your data is clustered by rownum, then this join should, in theory, be relatively fast
-- especially if it makes sense to exploit partitions.

-d 

On Apr 10, 2012, at 10:37 AM, <karanveer.singh@barclays.com> <karanveer.singh@barclays.com>
wrote:

> Makes sense but is not the distribution across nodes for a chunk of records in that order.

> 
> If Hive cannot help me do this, is there another way I can do this? I tried generating
an identifier using the perl script invoked using Hive but it does not seem to work fine.
While the stand alone script works fine, when the record is created in hive using std output
from perl - I see 2 records for some of the unique identifiers. I explored the possibility
of default data type changes but that does not solve the problem. 
> 
> Regards,
> Karan
> 
> 
> -----Original Message-----
> From: Philip Tromans [mailto:philip.j.tromans@gmail.com] 
> Sent: 10 April 2012 19:48
> To: user@hive.apache.org
> Subject: Re: Lag function in Hive
> 
> Hi Karan,
> 
> To the best of my knowledge, there isn't one. It's also unlikely to
> happen because it's hard to parallelise in a map-reduce way (it
> requires knowing where you are in a result set, and who your
> neighbours are and they in turn need to be present on the same node as
> you which is difficult to guarantee).
> 
> Cheers,
> 
> Phil.
> 
> On 10 April 2012 14:44,  <karanveer.singh@barclays.com> wrote:
>> Hi,
>> 
>> Is there something like a 'lag' function in HIVE? The requirement is to
>> calculate difference for the same column for every 2 subsequent records.
>> 
>> For example.
>> 
>> Row, Column A, Column B
>> 1, 10, 100
>> 2, 20, 200
>> 3, 30, 300
>> 
>> 
>> The result that I need should be like:
>> 
>> Row, Column A, Column B, Result
>> 1, 10, 100, NULL
>> 2, 20, 200, 100 (200-100)
>> 3, 30, 300, 100 (300-200)
>> 
>> Rgds,
>> Karan
>> 
>> 
>> 
>> 
>> 
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