Shravan Matthur Narayanamurthy commented on PIG-554:

1) Consider the following script:
A = load 'file1';
B = load 'file2';
C = filter A by $0>10;
D = filter B by $0<10;
E = join C by $0, D by $0 using replicated;

We need to materialize the result of D before we can use it as replicated 
input. Also DC has not been used as it doesn't support directories iirc (we 
will have to handle many complications manually) and the load specification in 
pig can contain regexps too. Also as the size of the replicated file is small 
it doesn't make too much diff.

2) Instead of writing all the code to handle the various combinations of the 
group item specification, I chose to use LR which already does it. I think I 
store only the plain tuple(extracted from the LR ouput) and not the LR output 
in the hashtables. So it doesn't add to any memory overhead. The LR is used 
only to separate out key & value and these are stored as a mapping from key to 
value (plain tuples).

> Fragment Replicate Join
> -----------------------
>                 Key: PIG-554
>                 URL: https://issues.apache.org/jira/browse/PIG-554
>             Project: Pig
>          Issue Type: New Feature
>    Affects Versions: types_branch
>            Reporter: Shravan Matthur Narayanamurthy
>            Assignee: Shravan Matthur Narayanamurthy
>             Fix For: types_branch
>         Attachments: frjofflat.patch, frjofflat1.patch
> Fragment Replicate Join(FRJ) is useful when we want a join between a huge 
> table and a very small table (fitting in memory small) and the join doesn't 
> expand the data by much. The idea is to distribute the processing of the huge 
> files by fragmenting it and replicating the small file to all machines 
> receiving a fragment of the huge file. Because of the availability of the 
> entire small file, the join becomes a trivial task without needing any break 
> in the pipeline. Exhaustive test have done to determine the improvement we 
> get out of FRJ. Here are the details: http://wiki.apache.org/pig/PigFRJoin
> The patch makes changes to parts of the code where new operators are 
> introduced. Currently, when a new operator is introduced, its alias is not 
> set. For schema computation I have modified this behaviour to set the alias 
> of the new operator to that of its predecessor. The logical side of the patch 
> mimics the cogroup behavior as join syntax closely resembles that of cogroup. 
> Currently, this patch doesn't have support for joins other than inner joins. 
> The rest of the code has been documented.

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