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Pig >> mail # user >> Can't JOIN self?


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Re: Can't JOIN self?
It isn't a bug that you need to declare the join twice in your script.  That is necessary for clarity and semantic correctness.  That is, if we allowed:

A = load 'bla';
B = join A by user, A by user;

then you'd have two user fields in the B with no way to disambiguate.  What's a bug (or missed optimization opportunity) is that we actually double read and shuffle the data.  We could optimize here and only read shuffle one copy and then do the join in the reduce.

Alan.

On Jul 20, 2012, at 12:53 AM, Dmitriy Ryaboy wrote:

> It's kind if a waste of io and mappers. If not a bug, it's an optimization opportunity.
>
> On Jul 19, 2012, at 10:34 PM, Bill Graham <[EMAIL PROTECTED]> wrote:
>
>> No, it isn't a bug as I see it. You need to load the two relations
>> separately because a join is across two separate data sources.
>>
>>
>> On Thu, Jul 19, 2012 at 10:10 PM, Russell Jurney
>> <[EMAIL PROTECTED]>wrote:
>>
>>> So it is a bug? Because Pig will not let me self JOIN. I have to LOAD the
>>> data twice.
>>>
>>> On Thu, Jul 19, 2012 at 9:49 PM, Bill Graham <[EMAIL PROTECTED]> wrote:
>>>
>>>> No, to Pig a self join is just like a regular join across two different
>>>> relations. It just happens to be to the same input data.
>>>>
>>>> On Thu, Jul 19, 2012 at 8:39 PM, Russell Jurney <[EMAIL PROTECTED]
>>>>> wrote:
>>>>
>>>>> Is this a bug?
>>>>>
>>>>> On Thu, Jul 19, 2012 at 8:00 PM, Robert Yerex <
>>>>> [EMAIL PROTECTED]> wrote:
>>>>>
>>>>>> The only way to get it to work is to load a second copy.
>>>>>>
>>>>>> On Thu, Jul 19, 2012 at 7:46 PM, Russell Jurney <
>>>>> [EMAIL PROTECTED]
>>>>>>> wrote:
>>>>>>
>>>>>>> Note: this works if I LOAD a new, 2nd relation and do the join.
>>>>>>>
>>>>>>> On Thu, Jul 19, 2012 at 7:34 PM, Russell Jurney <
>>>>>> [EMAIL PROTECTED]
>>>>>>>> wrote:
>>>>>>>
>>>>>>>> I have a problem where I can't join a relation to itself on a
>>>>> different
>>>>>>>> field.
>>>>>>>>
>>>>>>>> describe pairs
>>>>>>>> pairs: {from: chararray,to: chararray,message_id:
>>>>>> chararray,in_reply_to:
>>>>>>>> chararray}
>>>>>>>>
>>>>>>>> pairs2 = pairs;
>>>>>>>>
>>>>>>>> with_reply = join pairs by in_reply_to, pairs2 by message_id;
>>>>>>>>
>>>>>>>>
>>>>>>>> I get this error:
>>>>>>>>
>>>>>>>> 2012-07-19 19:31:16,927 [main] ERROR
>>>>> org.apache.pig.tools.grunt.Grunt -
>>>>>>>> ERROR 1200: Pig script failed to parse:
>>>>>>>> <line 20, column 6> pig script failed to validate:
>>>>>>>> org.apache.pig.impl.logicalLayer.FrontendException: ERROR 2225:
>>>>>>> Projection
>>>>>>>> with nothing to reference!
>>>>>>>> 2012-07-19 19:31:16,928 [main] ERROR
>>>>> org.apache.pig.tools.grunt.Grunt -
>>>>>>>> Failed to parse: Pig script failed to parse:
>>>>>>>> <line 20, column 6> pig script failed to validate:
>>>>>>>> org.apache.pig.impl.logicalLayer.FrontendException: ERROR 2225:
>>>>>>> Projection
>>>>>>>> with nothing to reference!
>>>>>>>> at
>>>>>>>>
>>>>>>
>>>> org.apache.pig.parser.QueryParserDriver.parse(QueryParserDriver.java:182)
>>>>>>>> at
>>>> org.apache.pig.PigServer$Graph.validateQuery(PigServer.java:1565)
>>>>>>>> at
>>>> org.apache.pig.PigServer$Graph.registerQuery(PigServer.java:1538)
>>>>>>>> at org.apache.pig.PigServer.registerQuery(PigServer.java:540)
>>>>>>>> at
>>>>>>>
>>>> org.apache.pig.tools.grunt.GruntParser.processPig(GruntParser.java:970)
>>>>>>>> at
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>> org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:386)
>>>>>>>> at
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>> org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:189)
>>>>>>>> at
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>> org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:165)
>>>>>>>> at org.apache.pig.tools.grunt.Grunt.run(Grunt.java:69)
>>>>>>>> at org.apache.pig.Main.run(Main.java:490)
>>>>>>>> at org.apache.pig.Main.main(Main.java:111)
>>>>>>>> at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)