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From "Santhosh Srinivasan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-984) PERFORMANCE: Implement a map-side group operator to speed up processing of ordered data
Date Thu, 01 Oct 2009 17:01:23 GMT

    [ https://issues.apache.org/jira/browse/PIG-984?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12761270#action_12761270

Santhosh Srinivasan commented on PIG-984:

bq. But this is in line with what we've done for joins, philosophically, semantically, and

Not exactly; with joins we are exposing different kinds of joins. Here we are exposing the
underlying aspects of the framework (mapside). If there is a parallel framework that does
not do map-reduce then having mapside in the language is philosophically and semantically
not correct.

> 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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