Pig doesn't do early projection for some scripts
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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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