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https://issues.apache.org/jira/browse/SOLR-3109?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13204862#comment-13204862
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Russell Black commented on SOLR-3109:
-------------------------------------
As the patch currently stands, someone who can guarantee that a group will
reside wholly on a single shard will benefit already because it will only send
the query the shard that contains the group of interest. There would be no
need to have a separate advanced option. I simply made the data structure
allow for the _possibility_ of having multiple shards per group, but there is
no additional overhead for the single-shard case.
> 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
> Fix For: 3.6, 4.0
>
> 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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