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MapReduce >> mail # user >> Re: How can I record some position of context in Reduce()?


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Re: How can I record some position of context in Reduce()?
Theta joins don't do we'll in any system.

Are both tables large?
If not its a map side join and the reducer will be just an ordinary reducer(s).
Sent from a remote device. Please excuse any typos...

Mike Segel

On Apr 11, 2013, at 12:18 AM, Vikas Jadhav <[EMAIL PROTECTED]> wrote:

> I wil express it in SQL form
>  
> select * from table1, table2 where table1.attr < table2.attr
>  
> it is also called theta join where theta can be <, >, <=,>=,!>  
>
>
> On Wed, Apr 10, 2013 at 9:35 PM, Michel Segel <[EMAIL PROTECTED]> wrote:
>> Not sure what is meant by a non equi join.
>>
>> Are you saying something like for every row in X, join it to all of the rows in Y where Y.a < something?
>>
>> Is that what you are suggesting?
>>
>>
>> Sent from a remote device. Please excuse any typos...
>>
>> Mike Segel
>>
>> On Apr 10, 2013, at 9:11 AM, Vikas Jadhav <[EMAIL PROTECTED]> wrote:
>>
>>> How are you going to support NON EQUI Join using MapReduce ?
>>> As per my understanding there is only one way to do this is
>>> to bring all data to one reducer then reducer will know lesser/greater
>>> values correctly.
>>> Correct me if I am wrong.
>>> Thank You.
>>>  
>>>   Regards,
>>>   Vikas
>>>  
>>>
>>>
>>> On Wed, Apr 10, 2013 at 4:22 PM, Michel Segel <[EMAIL PROTECTED]> wrote:
>>>> Can you show an example of your join?
>>>> All joins are an equality in that the key has to match.
>>>> Whether its a one to one , one to many, or many to many remains to be seen.
>>>>
>>>>
>>>> Sent from a remote device. Please excuse any typos...
>>>>
>>>> Mike Segel
>>>>
>>>> On Apr 9, 2013, at 10:35 AM, Effyroth Gu <[EMAIL PROTECTED]> wrote:
>>>>
>>>>> Only equality joins, outer joins, and left semi joins are supported in Hive. Hive does not support join conditions that are not equality conditions as it is very difficult to express such conditions as a map/reduce job. Also, more than two tables can be joined in Hive.
>>>>>
>>>>>
>>>>> 2013/4/9 Michael Segel <[EMAIL PROTECTED]>
>>>>>> Hi,
>>>>>>
>>>>>> Your cross join is supported in both pig and hive. (Cross, and Theta joins)
>>>>>>
>>>>>> So there must be code to do this.
>>>>>>
>>>>>> Essentially in the reducer you would have your key and then the set of rows that match the key. You would then perform the cross product on the key's result set and output them to the collector as separate rows.
>>>>>>
>>>>>> I'm not sure why you would need the reduce context.
>>>>>>
>>>>>> But then again, I'm still on my first cup of coffee. ;-)
>>>>>>
>>>>>>
>>>>>> On Apr 9, 2013, at 12:15 AM, Vikas Jadhav <[EMAIL PROTECTED]> wrote:
>>>>>>
>>>>>>> Hi
>>>>>>> I am also woring on join using MapReduce
>>>>>>> i think instead of finding postion of table in RawKeyValuIterator.
>>>>>>> what we can do modify context.write method to alway write key as table name or id
>>>>>>> then we dont need to find postion we can get Key and Value from "reducerContext"
>>>>>>>  
>>>>>>> befor calling reducer.run(reducerContext) in ReduceTask.java we can  add
>>>>>>> method join in Reducer.java Reducer class and give call to reducer.join(reduceContext)
>>>>>>>  
>>>>>>>  
>>>>>>> I just wonder how r going to support NON EQUI join.
>>>>>>>  
>>>>>>> I am also having same problem how to do join if datasets cant fit in to memory.
>>>>>>>  
>>>>>>>  
>>>>>>> for now i am cloning using following code :
>>>>>>>  
>>>>>>>  
>>>>>>> KEYIN key = context.getCurrentKey() ;
>>>>>>> KEYIN outKey = null;
>>>>>>> try {
>>>>>>>     outKey = (KEYIN)key.getClass().newInstance();
>>>>>>>    }
>>>>>>> catch(Exception e)
>>>>>>>  {}        
>>>>>>> ReflectionUtils.copy(context.getConfiguration(), key, outKey);      
>>>>>>>
>>>>>>>  Iterable<VALUEIN> values = context.getValues();
>>>>>>>  ArrayList<VALUEIN> myValues = new ArrayList<VALUEIN>();
>>>>>>>  for(VALUEIN value: values) {        
>>>>>>>    VALUEIN outValue = null;
>>>>>>>     try {
>>>>>>>          outValue = (VALUEIN)value.getClass().newInstance();