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From "Alan Gates (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 16:31:23 GMT

    [ https://issues.apache.org/jira/browse/PIG-984?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12761257#action_12761257

Alan Gates commented on PIG-984:

The controlling philosophic point here is that pigs are domestic animals (see http://wiki.apache.org/pig/PigPhilosophy).
 Just as in join, where we have exposed all possible join implementations to the user, we
want to do the same with this new feature.  At some future point when we have a capable optimizer,
we will try to select the best type of join, and try to select this form of grouping when
it's appropriate.  But even then, we want to expose this functionality to the user directly
because the optimizer may not have access to the necessary information to determine the best
grouping choice (e.g., data sources with no schema).  And we don't want to wait until the
optimizer can handle these things to start exposing it.  

I don't agree with Santosh's assertion that the language is evolving with no definition. 
I agree we do not yet have a comprehensive definition of Pig Latin, which we need.  But this
is in line with what we've done for joins, philosophically, semantically, and syntacticly.

> 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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