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From "Mridul Muralidharan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-807) PERFORMANCE: Provide a way for UDFs to use read-once bags (backed by the Hadoop values iterator)
Date Tue, 19 May 2009 22:07:45 GMT

    [ https://issues.apache.org/jira/browse/PIG-807?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12710901#action_12710901

Mridul Muralidharan commented on PIG-807:

I think I am missing something here.

If I did not get it wrong, two (different ?) usecases seem to be mentioned here ?

1) Avoid materializing bag's for a record when it can be streamed from the underlying data.
bag's currently created through (co)group output seems to fall inside this.
As in :
B = GROUP A by id;
C = FOREACH B generate SUM($1.field);

This does not reqiure the $1.field bag to be created explicitly - but through an iterator
interface, just stream the values from underlying reducer output.

2) The group ALL based construct seem to be to directly stream an entire relation through
As a shorthand for 
A_tmp = GROUP A all;
B = FOREACH A_tmp GENERATE algUdf($1);

If I am right in splitting this, then :

First usecase has tremendous potential for improving performance - particularly to remove
the annoying OOM's or spills which happen : but not sure how it interact with pig's current
pipeline design... (if any).

Since there are alternatives (though more cryptic) to do it, I dont have any particular opinion
about 2.


> PERFORMANCE: Provide a way for UDFs to use read-once bags (backed by the Hadoop values
> ------------------------------------------------------------------------------------------------
>                 Key: PIG-807
>                 URL: https://issues.apache.org/jira/browse/PIG-807
>             Project: Pig
>          Issue Type: Improvement
>    Affects Versions: 0.2.1
>            Reporter: Pradeep Kamath
>             Fix For: 0.3.0
> Currently all bags resulting from a group or cogroup are materialized as bags containing
all of the contents. The issue with this is that if a particular key has many corresponding
values, all these values get stuffed in a bag which may run out of memory and hence spill
causing slow down in performance and sometime memory exceptions. In many cases, the udfs which
use these bags coming out a group and cogroup only need to iterate over the bag in a unidirectional
read-once manner. This can be implemented by having the bag implement its iterator by simply
iterating over the underlying hadoop iterator provided in the reduce. This kind of a bag is
also needed in http://issues.apache.org/jira/browse/PIG-802. So the code can be reused for
this issue too. The other part of this issue is to have some way for the udfs to communicate
to Pig that any input bags that they need are "read once" bags . This can be achieved by having
an Interface - say "UsesReadOnceBags " which is serves as a tag to indicate the intent to
Pig. Pig can then rewire its execution plan to use ReadOnceBags is feasible.

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