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https://issues.apache.org/jira/browse/SOLR-3109?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13204533#comment-13204533
 ] 

Russell Black edited comment on SOLR-3109 at 2/9/12 4:06 PM:
-------------------------------------------------------------

The current {{TestDistributedGrouping}} test case is constructed in such a way 
that each record has a unique value for it's search group field ({{i1}}), so 
that there is never more than one record in any given search group.  This style 
of indexing conforms to the restriction discussed earlier.  This is likely the 
reason there were no test failures.  
                
      was (Author: rblack):
    The test case is constructed in such a way that each record has a unique 
value for it's search group field ({{i1}}), so that there is never more than 
one record in any given search group.  This style of indexing conforms to the 
restriction discussed earlier.  This is likely the reason there were no test 
failures.  
                  
> group=true requests result in numerous redundant shard requests
> ---------------------------------------------------------------
>
>                 Key: SOLR-3109
>                 URL: https://issues.apache.org/jira/browse/SOLR-3109
>             Project: Solr
>          Issue Type: Bug
>          Components: search
>    Affects Versions: 3.5, 4.0
>         Environment: 64-bit Linux, sharded environment
>            Reporter: Russell Black
>            Assignee: Martijn van Groningen
>            Priority: Critical
>              Labels: patch, performance
>         Attachments: SOLR-3109.patch, SOLR-3109.patch, SOLR-3109.patch
>
>
> During the second phase of a group query, the collator sends a query to each 
> of the shards.  The purpose of this query is for shards to respond with the 
> doc ids that match the set of group ids returned from the first phase.  The 
> problem is that it sends this second query to each shard multiple times.  
> Specifically, in an environment with n shards, each shard will be hit with an 
> identical query n times during the second phase of query processing, 
> resulting in O(_n_ ^2^) performance where _n_ is the number of shards.
> I have traced this bug down to a single line in 
> {{TopGroupsShardRequestFactory.java}}, and I am attaching a patch. 

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