[jira] Commented: (PIG-350) PERFORMANCE: Join optimization for pipeline rework

["Pradeep Kamath (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PIG-350?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12641460#action_12641460 ] 

Pradeep Kamath commented on PIG-350:
------------------------------------

Submitted patch with modification to Join4.patch
- The last input is now streamed instead of the first input. This is because we now have tuples arriving in package in the order of the index starting from input 0 upto input n. All tuples for a given index (input) arrive before any tuple for the next index.
- The last input is now sent as a tuple at a time to foreach rather than as a bag containing one tuple at a time
- A new optimized foreach has been introduced which assumes its input has already been "attached" (which will be the case in the join optimization)
- In POForeach all data structures (arrays) are now created statically and reused in each getNext() call rather than being re-created on each getNext() call. Likewise the datastructures in POJoinPackage are also created once up front and reused.


> PERFORMANCE: Join optimization for pipeline rework
> --------------------------------------------------
>
>                 Key: PIG-350
>                 URL: https://issues.apache.org/jira/browse/PIG-350
>             Project: Pig
>          Issue Type: Bug
>          Components: impl
>    Affects Versions: types_branch
>            Reporter: Alan Gates
>            Assignee: Pradeep Kamath
>             Fix For: types_branch
>
>         Attachments: join.patch, join2.patch, join3.patch, join4.patch, PIG-350.patch
>
>
> Currently, joins in pig are done as groupings where each input is grouped on the join key.  In the reduce phase, records from each input are collected into a bag for each key, and then a cross product done on these bags.  This can be optimized by selecting one (hopefully the largest) input and streaming through it rather than placing the results in a bag.  This will result in better memory usage, less spills to disk due to bag overflow, and better performance.  Ideally, the system would intelligently select which input to stream, based on a histogram of value distributions for the keys.  Pig does not have that kind of metadata.  So for now it is best to always pick the same input (first or last) so that the user can select which input to stream.
> Similarly, order by in pig is done in this same way, with the grouping keys being the ordering keys, and only one input.  In this case pig still currently collects all the records for a key into a bag, and then flattens the bag.  This is a total waste, and in some cases causes significant performance degradation.  The same optimization listed above can address this case, where the last bag (in this case the only bag) is streamed rather than collected.
> To do these operations, a new POJoinPackage will be needed.  It will replace POPackage and the following POForEach in these types of scripts, handling pulling the records from hadoop and streaming them into the pig pipeline.  A visitor will need to be added in the map reduce compilation phase that detects this case and combines the POPackage with POForeach into this new POJoinPackage.

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