[jira] [Created] (SOLR-8297) Allow join query over 2 sharded collections: enhance functionality and exception handling

["Paul Blanchaert (JIRA)" <ji...@apache.org>]

Paul Blanchaert created SOLR-8297:
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             Summary: Allow join query over 2 sharded collections: enhance functionality and exception handling
                 Key: SOLR-8297
                 URL: https://issues.apache.org/jira/browse/SOLR-8297
             Project: Solr
          Issue Type: Improvement
          Components: SolrCloud
    Affects Versions: 5.3
            Reporter: Paul Blanchaert


Enhancement based on SOLR-4905. New Jira issue raised as suggested by Mikhail Khludnev.
A) exception handling:
The exception "SolrCloud join: multiple shards not yet supported" thrown in the function findLocalReplicaForFromIndex of JoinQParserPlugin is not triggered correctly: In my use-case, I've a join on a facet.query and when my results are only found in 1 shard and the facet.query with the join is querying the last replica of the last slice, then the exception is not thrown.
I believe it's better to verify the nr of slices when we want to verify the  "multiple shards not yet supported" exception (so exception is thrown when zkController.getClusterState().getSlices(fromIndex).size()>1).

B) functional enhancement:
I would expect that there is no problem to perform a cross-core join over sharded collections when the following conditions are met:
1) both collections are sharded with the same replicationFactor and numShards
2) router.field of the collections is set to the same "key-field" (collection of "fromindex" has router.field = "from" field and collection joined to has router.field = "to" field)

The router.field setup ensures that documents with the same "key-field" are routed to the same node. 
So the combination based on the "key-field" should always be available within the same node.

>From a user perspective, I believe these assumptions seem to be a "normal" use-case in the cross-core join in SolrCloud.

Hope this helps



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