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From "Richard Ding (JIRA)" <j...@apache.org>
Subject [jira] Created: (PIG-984) PERFORMANCE: Implement a map-side group operator to speed up processing of ordered data
Date Wed, 30 Sep 2009 19:09:23 GMT
PERFORMANCE: Implement a map-side group operator to speed up processing of ordered data 

                 Key: PIG-984
                 URL: https://issues.apache.org/jira/browse/PIG-984
             Project: Pig
          Issue Type: New Feature
            Reporter: Richard Ding

The general group by operation in Pig needs both mappers and reducers (the aggregation is
done in reducers). This incurs disk writes/reads  between mappers and reducers.

However, in the cases where the input data has the following properties

   1. The records with the same key are grouped together (such as the data is sorted by the
   2. The records with the same key are in the same mapper input.

the group by operation can be performed in the mappers only and thus remove the overhead of
disk writes/reads.

Alan proposed adding a hint to the group by clause like this one:

A = load 'input' using SomeLoader(...);
B = group A by $0 using "mapside";
C = foreach B generate ...

The proposed addition of using "mapside" to group will be a mapside group operator that collects
all records for a given key into a buffer. When it sees a key change it will emit the key
and bag for records it had buffered. It will assume that all keys for a given record are collected
together and thus there is not need to buffer across keys. 

It is expected that "SomeLoader" will be implemented by data systems such as Zebra to ensure
the data emitted by the loader satisfies the above properties (1) and (2).

It will be the responsibility of the user (or the loader) to guarantee these properties (1)
& (2) before invoking the mapside hint for the group by clause. The Pig runtime can't
check for the errors in the input data.

For the group by clauses with mapside hint, Pig Latin will only support group by columns (including
*), not group by expressions nor group all. 

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