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From "David Ciemiewicz (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 17:10:45 GMT

    [ https://issues.apache.org/jira/browse/PIG-807?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12710798#action_12710798

David Ciemiewicz commented on PIG-807:

Going back to my early days of Pig and the confusion / aversion people have to "group all",
if we head ReadOnce bags, there would often be absolutely no need for "group all" or any form
of mapping step. 

In other words, it would really be useful if people could just write the following summary
code without using an intervening group ... all.

It would make it easier for people to transition from SQL to Pig.  It make Pig a little bit
higher level language than it is.

A = load ... 

B = foreach A generate 

> 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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