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From "Joydeep Sen Sarma (JIRA)" <>
Subject [jira] Commented: (HIVE-223) when using map-side aggregates - perform single map-reduce group-by
Date Mon, 02 Mar 2009 19:46:56 GMT


Joydeep Sen Sarma commented on HIVE-223:

+      // In case merge is called without an iterate, the 
+      if (pos == -1) {
+        mSum += Double.parseDouble(o);
+        mCount ++;
+      } 

assuming this happens when iterate is called with null - is it correct to include the null
entry in the count?

I did some search on web (for ex:
- it suggests that AVG definition skips nulls ..

anyway - this is inconsistent with the way the iterate is coded (the iterate doesn't bump
count for nulls)

> when using map-side aggregates - perform single map-reduce group-by
> -------------------------------------------------------------------
>                 Key: HIVE-223
>                 URL:
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Joydeep Sen Sarma
>            Assignee: Namit Jain
>         Attachments: 223.2.txt, 223.3.txt, 223.patch1.txt, patch.txt
> today even when we do map side aggregates - we do multiple map-reduce jobs. however -
the reason for doing multiple map-reduce group-bys (for single group-bys) was the fear of
skews. When we are doing map side aggregates - skews should not exist for the most part. There
can be two reason for skews:
> - large number of entries for a single grouping set - map side aggregates should take
care of this
> - badness in hash function that sends too much stuff to one reducer - we should be able
to take care of this by having good hash functions (and prime number reducer counts)
> So i think we should be able to do a single stage map-reduce when doing map-side aggregates.

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