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

{code} 
A = load ... 

B = foreach A generate 
COUNT(*), 
SUM(m), 
SUM(n), 
SUM(o), 
SUM(p). 
SUM(q), 
SUM(r), 
... 
{code} 

> PERFORMANCE: Provide a way for UDFs to use read-once bags (backed by the 
> Hadoop values iterator)
> ------------------------------------------------------------------------------------------------
>
>                 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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