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From Mridul Muralidharan <mrid...@yahoo-inc.com>
Subject Re: [jira] Created: (PIG-350) Join optimization for pipeline rework
Date Thu, 31 Jul 2008 21:48:49 GMT

This would be absolutely great !
Btw, hope this continues to work fine with combiners in case of COGROUP 
+ FILTER (combiners are applicable in this case right ? or only for 
group ?).

Additionally, what would the impact of this be on CROSS + FILTER ? (I am 
assuming that CROSS + FILTER is not combinable currently)


Alan Gates (JIRA) wrote:
> 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: Daniel Dai
>             Priority: Critical
>              Fix For: types_branch
> 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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