PERFORMANCE: multi-query optimization on multiple group bys following a join or
cogroup
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Key: PIG-983
URL: https://issues.apache.org/jira/browse/PIG-983
Project: Pig
Issue Type: Improvement
Components: impl
Reporter: Richard Ding
The current multi-query optimizer works well with pig scripts like this one:
{code}
data = LOAD 'input' AS (a:chararray, b:int, c:int);
A = GROUP data BY b;
B = GROUP data BY c;
C = FOREACH A GENERATE group, COUNT(data);
D = FOREACH B GENERATE group, SUM(data.b);
STORE C INTO 'output1';
STORE D INTO 'output2';
{code}
In this case the original three Map-Reduce jobs are merged into one MR job by
the optimizer.
The current optimizer, however, won't reduce the number of MR jobs for the
scripts in which multiple group bys follow a join or a cogroup, such as this
one:
{code}
data1 = LOAD 'input1' AS (a1:chararray, b1:int, c1:int);
data2 = LOAD 'input2' AS (a2:chararray, b2:int, c2:int);
A = JOIN data1 BY a1, data2 BY a2;
B = GROUP A BY data1::b1;
C = GROUP B BY data2::c2;
D = FOREACH B GENERATE group, COUNT(A);
E = FOREACH C GENERATE group, SUM(A.data2::b2);
STORE D INTO 'output1';
STORE E INTO 'output2';
{code}
Three MR jobs are still needed to run this script.
Multi-query optimizer should work with this kind of scripts by merging the
group bys and reducing the overall MR jobs.
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