group=true requests result in numerous redundant shard requests
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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
Priority: Critical
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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