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https://issues.apache.org/jira/browse/SOLR-16717?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17739621#comment-17739621
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Ilan Ginzburg commented on SOLR-16717:
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Left a few comments on the
[PR|http://example.com|https://github.com/apache/solr/pull/1691].
Copying the most structuring one here (because it's more a design level comment
than a code level comment):
{quote}The feature does not really require a one to one matching of shards.
Assuming the matching shards do have the same ranges, this is just one setup
that allows doing a local join.
What's really needed is to have "from" replicas covering the range of "to"
replicas on every node where there are "to" replicas, regardless of the
topology of the collections (sharded or not sharded, different number of shards
for each collections).
Verifying such a hash range coverage generalizes the existing join (in which
the "from" collection must be single shard and have a replica on each nodes
that has a "to" replica) and the more advanced join you're implementing here.
Verifying by range will also support the use case of shard splits on a node, as
the number of shards of each collection is not a criteria for allowing or
disallowing the join, and a split does not change the total range of a
collection present on a node.{quote}
> Joining collocated shards
> -------------------------
>
> Key: SOLR-16717
> URL: https://issues.apache.org/jira/browse/SOLR-16717
> Project: Solr
> Issue Type: Improvement
> Reporter: Mikhail Khludnev
> Assignee: Mikhail Khludnev
> Priority: Major
> Fix For: 9.3
>
> Time Spent: 40m
> Remaining Estimate: 0h
>
> h3. Context
> It's about \{!join} query parser and distributed mode.
> h3. As is
> SOLR-4905 allows to join from single shard collection to many shards
> collection.
> h3. Challenge
> * Support multiple shards on from side as well,
> * but strictly stick to collocated indices that promise much performance.
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