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https://issues.apache.org/jira/browse/PIG-1637?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12915950#action_12915950
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Daniel Dai commented on PIG-1637:
---------------------------------
Yes, it could be improved as per Xuefu's suggestion. Anyway, current patch
solve the "combiner not used" issue, will commit this part first. I will open
another Jira to improve it. Also, MergeForEach is a best example to practice
cloning framework [PIG-1587|https://issues.apache.org/jira/browse/PIG-1587], so
it is better to improve it once PIG-1587 is available.
> Combiner not use because optimizor inserts a foreach between group and
> algebric function
> ----------------------------------------------------------------------------------------
>
> Key: PIG-1637
> URL: https://issues.apache.org/jira/browse/PIG-1637
> Project: Pig
> Issue Type: Bug
> Affects Versions: 0.8.0
> Reporter: Daniel Dai
> Assignee: Daniel Dai
> Fix For: 0.8.0
>
> Attachments: PIG-1637-1.patch, PIG-1637-2.patch
>
>
> The following script does not use combiner after new optimization change.
> {code}
> A = load ':INPATH:/pigmix/page_views' using
> org.apache.pig.test.udf.storefunc.PigPerformanceLoader()
> as (user, action, timespent, query_term, ip_addr, timestamp,
> estimated_revenue, page_info, page_links);
> B = foreach A generate user, (int)timespent as timespent,
> (double)estimated_revenue as estimated_revenue;
> C = group B all;
> D = foreach C generate SUM(B.timespent), AVG(B.estimated_revenue);
> store D into ':OUTPATH:';
> {code}
> This is because after group, optimizer detect group key is not used
> afterward, it add a foreach statement after C. This is how it looks like
> after optimization:
> {code}
> A = load ':INPATH:/pigmix/page_views' using
> org.apache.pig.test.udf.storefunc.PigPerformanceLoader()
> as (user, action, timespent, query_term, ip_addr, timestamp,
> estimated_revenue, page_info, page_links);
> B = foreach A generate user, (int)timespent as timespent,
> (double)estimated_revenue as estimated_revenue;
> C = group B all;
> C1 = foreach C generate B;
> D = foreach C1 generate SUM(B.timespent), AVG(B.estimated_revenue);
> store D into ':OUTPATH:';
> {code}
> That cancel the combiner optimization for D.
> The way to solve the issue is to merge the C1 we inserted and D. Currently,
> we do not merge these two foreach. The reason is that one output of the first
> foreach (B) is referred twice in D, and currently rule assume after merge, we
> need to calculate B twice in D. Actually, C1 is only doing projection, no
> calculation of B. Merging C1 and D will not result calculating B twice. So C1
> and D should be merged.
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