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From Chen Song <chen.song...@gmail.com>
Subject Re: map side join with group by
Date Thu, 13 Dec 2012 19:12:25 GMT
Nitin

Yeah. My original question is that is there a way to force Hive (or rather
to say, is it possible) to execute map side join at mapper phase and group
by in reduce phase. So instead of launching a map only job (join) and map
reduce job (group by), doing it altogether in a single MR job. This is
obviously not what Hive does but I am wondering if it is a nice feature to
have.

The point you made (different keys in join and group by) only matters when
it is the time in reduce phase, right? As map side join takes care of join
at mapper phase, it sounds to me natural that group by can be done in the
reduce phase in the same job. The only hassle that I can think of is that
map output have to be resorted (based on group by keys).

Chen

On Thu, Dec 13, 2012 at 1:42 PM, Nitin Pawar <nitinpawar432@gmail.com>wrote:

> chen in mapside join .. there are no reducers .. its MAP ONLY job
>
>
> On Thu, Dec 13, 2012 at 11:54 PM, Chen Song <chen.song.82@gmail.com>wrote:
>
>> Understood that fact that it is impossible in the same MR job if both
>> join and group by are gonna happen in the reduce phase (because the join
>> keys and group by keys are different). But for map side join, the joins
>> would be complete by the end of the map phase, and outputs should be ready
>> to be distributed to reducers based on group by keys.
>>
>> Chen
>>
>>
>> On Thu, Dec 13, 2012 at 11:04 AM, Nitin Pawar <nitinpawar432@gmail.com>wrote:
>>
>>> Thats because for the first job the join keys are different and second
>>> job group by keys are different, you just cant assume join keys and group
>>> keys will be same so they are two different jobs
>>>
>>>
>>> On Thu, Dec 13, 2012 at 8:26 PM, Chen Song <chen.song.82@gmail.com>wrote:
>>>
>>>> Yeah, my abridged version of query might be a little broken but my
>>>> point is that when a query has a map join and group by, even in its
>>>> simplified incarnation, it will launch two jobs. I was just wondering why
>>>> map join and group by cannot be accomplished in one MR job.
>>>>
>>>> Best,
>>>> Chen
>>>>
>>>>
>>>> On Thu, Dec 13, 2012 at 12:30 AM, Nitin Pawar <nitinpawar432@gmail.com>wrote:
>>>>
>>>>> I think Chen wanted to know why this is two phased query if I
>>>>> understood it correctly
>>>>>
>>>>> When you run a mapside join .. it just performs the join query ..
>>>>> after that to execute the group by part it launches the second job.
>>>>> I may be wrong but this is how I saw it whenever I executed group by
>>>>> queries
>>>>>
>>>>>
>>>>> On Thu, Dec 13, 2012 at 7:11 AM, Mark Grover <
>>>>> grover.markgrover@gmail.com> wrote:
>>>>>
>>>>>> Hi Chen,
>>>>>> I think we would need some more information.
>>>>>>
>>>>>> The query is referring to a table called "d" in the MAPJOIN hint
but
>>>>>> there is not such table in the query. Moreover, Map joins only make
>>>>>> sense when the right table is the one being "mapped" (in other words,
>>>>>> being kept in memory) in case of a Left Outer Join, similarly if
the
>>>>>> left table is the one being "mapped" in case of a Right Outer Join.
>>>>>> Let me know if this is not clear, I'd be happy to offer a better
>>>>>> explanation.
>>>>>>
>>>>>> In your query, the where clause on a column called "hour", at this
>>>>>> point I am unsure if that's a column of table1 or table2. If it's
>>>>>> column on table1, that predicate would get pushed up (if you have
>>>>>> hive.optimize.ppd property set to true), so it could possibly be
done
>>>>>> in 1 MR job (I am not sure if that's presently the case, you will
have
>>>>>> to check the explain plan). If however, the where clause is on a
>>>>>> column in the right table (table2 in your example), it can't be pushed
>>>>>> up since a column of the right table can have different values before
>>>>>> and after the LEFT OUTER JOIN. Therefore, the where clause would
need
>>>>>> to be applied in a separate MR job.
>>>>>>
>>>>>> This is just my understanding, the full proof answer would lie in
>>>>>> checking out the explain plans and the Semantic Analyzer code.
>>>>>>
>>>>>> And for completeness, there is a conditional task (starting Hive
0.7)
>>>>>> that will convert your joins automatically to map joins where
>>>>>> applicable. This can be enabled by enabling hive.auto.convert.join
>>>>>> property.
>>>>>>
>>>>>> Mark
>>>>>>
>>>>>> On Wed, Dec 12, 2012 at 3:32 PM, Chen Song <chen.song.82@gmail.com>
>>>>>> wrote:
>>>>>> > I have a silly question on how Hive interpretes a simple query
with
>>>>>> both map
>>>>>> > side join and group by.
>>>>>> >
>>>>>> > Below query will translate into two jobs, with the 1st one as
a map
>>>>>> only job
>>>>>> > doing the join and storing the output in a intermediary location,
>>>>>> and the
>>>>>> > 2nd one as a map-reduce job taking the output of the 1st job
as
>>>>>> input and
>>>>>> > doing the group by.
>>>>>> >
>>>>>> > SELECT
>>>>>> > /*+ MAPJOIN(d) */
>>>>>> > table.a, sum(table2.b)
>>>>>> > from table
>>>>>> > LEFT OUTER JOIN table2
>>>>>> > ON table.id = table2.id
>>>>>> > where hour = '2012-12-11 11'
>>>>>> > group by table.a
>>>>>> >
>>>>>> > Why can't this be done within a single map reduce job? As what
I
>>>>>> can see
>>>>>> > from the query plan is that all 2nd job mapper do is taking
the 1st
>>>>>> job's
>>>>>> > mapper output.
>>>>>> >
>>>>>> > --
>>>>>> > Chen Song
>>>>>> >
>>>>>> >
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Nitin Pawar
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Chen Song
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Nitin Pawar
>>>
>>
>>
>>
>> --
>> Chen Song
>>
>>
>>
>
>
> --
> Nitin Pawar
>



-- 
Chen Song

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