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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-931) Sorted Group By
Date Wed, 02 Dec 2009 06:50:20 GMT

    [ https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12784660#action_12784660
] 

Namit Jain commented on HIVE-931:
---------------------------------

 } else if (node instanceof exprNodeGenericFuncDesc) { 	
				158 		exprNodeGenericFuncDesc udfNode = ((exprNodeGenericFuncDesc)node); 	
				159 		GenericUDF udf = udfNode.getGenericUDF(); 	
				160 		if(!FunctionRegistry.isDeterministic(udf)) 	
				161 		return; 	
				162 		groupByKeys.addAll(0, udfNode.getChildExprs());


Isnt there a bug here ?

group by foo(foo2(x))


You cant assume that the first child is a column - you should recurse till you get a column


> Sorted Group By
> ---------------
>
>                 Key: HIVE-931
>                 URL: https://issues.apache.org/jira/browse/HIVE-931
>             Project: Hadoop Hive
>          Issue Type: New Feature
>          Components: Query Processor
>            Reporter: Namit Jain
>            Assignee: He Yongqiang
>             Fix For: 0.5.0
>
>         Attachments: hive-931-2009-11-18.patch, hive-931-2009-11-19.patch, hive-931-2009-11-20.3.patch,
hive-931-2009-11-21.patch, hive-931-2009-12-01.patch
>
>
> If the table is sorted by a given key, we don't use that for group by. That can be very
useful.
> For eg: if T is sorted by column c1,
> For select c1, aggr() from T group by c1
> we always use a single map-reduce job. No hash table is needed on the mapper, since the
data is sorted by c1 anyway.
> This will reduce the memory pressure on the mapper and also remove overhead of maintaining
the hash table.

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