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https://issues.apache.org/jira/browse/PIG-984?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12761070#action_12761070
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Dmitriy V. Ryaboy commented on PIG-984:
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Good idea.

It should be straightforward to look at the sort info associated with the 
ResourceSchema (see the load/store proposal) to know whether the data is 
sorted; this frees us from relying on loaders, lets us follow ORDER BYs and 
LIMITs, etc.

Still, this is not quite safe unless you know that the distribution key is a 
subset of your group key.  A simple sorted input stream can still be split 
among mappers with some rows with the same key going to one, and some to the 
other.  Do you have thoughts on how to handle such cases?

This is something that can be inferred looking at the schema and distribution 
key. I understand wanting a manual handle to turn on the behavior while 
developing, but the production version of this can be done automatically ( "if 
distributed by and sorted on a subset of group keys, apply map-side group" rule 
in the optimizer).

> 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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