Tom Winch created SOLR-8234:
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             Summary: Federated Search (new) - DJoin
                 Key: SOLR-8234
                 URL: https://issues.apache.org/jira/browse/SOLR-8234
             Project: Solr
          Issue Type: New Feature
            Reporter: Tom Winch
            Priority: Minor
             Fix For: 4.10.3


This issue describes a MergeStrategy implementation to facilitate federated 
search - that is, distributed search over documents stored in separated 
instances of SOLR (for example, one server per continent), where a single 
document (identified by an agreed, common unique id) may be stored in more than 
one server instance, with (possibly) differing fields and data.

When the MergeStrategy is used in a request handler (via the included QParser) 
in combination with distributed search (shards=), documents having an id that 
has already been seen are not discarded (as per the default behaviour) but, 
instead, are collected and returned as a group of documents all with the same 
id taking a single position in the result set (this is implemented using 
parent/child documents).

Documents are sorted in the result set based on the highest ranking document 
with the same id. It is possible for a document ranking high in one shard to 
rank very low on another shard. As a consequence of this, all shards must be 
asked to return the fields of for every document id in the result set (not just 
of those documents they returned), so that all the component parts of each 
document in the search result set are returned.

This issue combines with others to provide full federated search support. See 
also...

--

Note that this is a new implementation of federated search as opposed to the 
older issues SOLR-3799 to 3805.



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