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Date: Sun, 4 Dec 2011 06:32:40 +0000 (UTC)
From: "Daniel Dai (Resolved) (JIRA)" <jira@apache.org>
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Subject: [jira] [Resolved] (PIG-2395) Pig doesn't do early projection for
 some scripts
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     [ https://issues.apache.org/jira/browse/PIG-2395?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Daniel Dai resolved PIG-2395.
-----------------------------

    Resolution: Duplicate

This is a duplication of PIG-1324. It involves some significant rework of column prune optimizer.
                
> Pig doesn't do early projection for some scripts
> ------------------------------------------------
>
>                 Key: PIG-2395
>                 URL: https://issues.apache.org/jira/browse/PIG-2395
>             Project: Pig
>          Issue Type: Improvement
>    Affects Versions: 0.9.1
>         Environment: Linux
>            Reporter: Jie Li
>
> For some scripts Pig doesn't do early projection, e.g. dropping non-necessary fields as soon as possible. This is observed in two ways: 1) the output doesn't contain INFO like "ColumnPruneVisitor - Columns pruned for xxx: $0, $1"; 2) the job has as much or more local IO (see FILE_BYTES_READ and FILE_BYTES_WRITTEN) as HDFS IO (see HDFS_BYTES_READ).
> One example where Pig should figure out A's fields c~n can be dropped before the COGROUP.
> A = load '/tmp/A' USING PigStorage('|') as (a,b,c,d,e,f,g,h,i,j,k,l,m,n);
> B = load '/tmp/B' USING PigStorage('|') as (a);
> COG = cogroup A by a, B by a;
> out = foreach COG generate SUM(A.b) as sum;
> store out into '/tmp/out' USING PigStorage('|');
> Another similar example involves a GROUP operator.
> While Pig is able and assumed to do early projection in most cases, this inconsistency hurts the performance badly. 

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