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https://issues.apache.org/jira/browse/PIG-490?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12863417#action_12863417
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Alan Gates commented on PIG-490:
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In the current system, a fair amount, just because the code in the combiner
optimizer is so byzantine. I was hoping we could instead reuse the code from
the logical optimizer rewrite and thus make it much easier to make this change.
> Combiner not used when group elements referred to in tuple notation instead
> of flatten.
> ---------------------------------------------------------------------------------------
>
> Key: PIG-490
> URL: https://issues.apache.org/jira/browse/PIG-490
> Project: Pig
> Issue Type: Bug
> Affects Versions: 0.2.0
> Reporter: Alan Gates
> Fix For: 0.8.0
>
>
> Given a query like:
> {code}
> A = load 'myfile';
> B = group A by ($0, $1);
> C = foreach B generate group.$0, group.$1, COUNT(A);
> {code}
> The combiner will not be invoked. But if the last line is changed to:
> {code}
> C = foreach B generate flatten(group), COUNT(A);
> {code}
> it will be. The reason for the discrepancy is because the CombinerOptimizer
> checks that all of the projections are simple. If not, it does not use the
> combiner. group.$0 is not a simple projection, so this is failed. However,
> this is a common enough case that the CombinerOptimizer should detect it and
> still use the combiner.
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