Santhosh Srinivasan commented on PIG-984:

A couple of things:

1. I am concerned about extending the language for supporting features that can 
be handled internally. The scope of the language has not been defined but the 
language continues to evolve.

2. I agree with Thejas' comment about allowing expressions that do not alter 
the property. Pig will not be able to check that but it is no different from 
being able to check if the data is sorted or not.

> 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 keys).
>    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:
> {code}
> A = load 'input' using SomeLoader(...);
> B = group A by $0 using "mapside";
> C = foreach B generate ...
> {code}
> 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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