Allow join query over 2 sharded collections: enhance functionality and exception handling

Proposal

General Idea

Approach shikhasomani's range check algorithm to the most cases

Join behavior depending on router types of joined collections

CompositeId to CompositeId join behaviour for certain number of shards

Rules from the tables above

• joining from/to CompositeId and Implicit is blindly allowed, it pick ups any collocated replica, because users who do that probably understand what they do.
• when both sides are Implicit let's join shards by name. ie if request hits collectionTO_shardY_replica2 at a node, the collocated collectionFROM_shardY_replica* is expected.
• when both sides are CompositeId
  • from single shard to single shard - nobrainer, just needs collocated replica;
  • from multiple shards to single shard - all "from" shards (any it's replicas) are picked for joining
  • from single shard to multiple shards - existing SOLR-4905 functionality
  • from multiple to multiple - generic range check algorithm
    1. check that fromField and toField are router.keys in these collections
    2. take shard range for the current "to" collection replica (keep in mind that request is distributed across "to" collection shards)
    3. enumerate "from" collection shrads, find their subset which covers "to" shard range (this allows to handle any number of shards at both sides)
    4. pickup collocated replicas of these "from" shard subset

Caveat

this is quite sensitive to shard allocation (and/or replica placement) ie failed "from" replica cannot be collocated with the required "to" shard.

Initial Description

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