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https://issues.apache.org/jira/browse/SOLR-7584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14790545#comment-14790545
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Dennis Gove commented on SOLR-7584:
-----------------------------------
This supports joining any incoming set of streams. If you have a FacetStream
instance (SOLR-7903) then you could absolutely join it with some other stream
instance.
Due to current use of merge-join style it is a requirement that the incoming
streams be sorted in a similar order. That said, a hash-join style can
relatively easily be added in which case the ordering requirement will go away.
I think a hash-join would make a lot of sense for a FacetStream (or really any
kind of aggregation stream).
Using the feature added in SOLR-7669 (Add SelectStream to Streaming API) you
will be able to apply functions (called operations in that ticket) on the
joined data. Currently the only included operation
> Add Joins to the Streaming API and Streaming Expressions
> --------------------------------------------------------
>
> Key: SOLR-7584
> URL: https://issues.apache.org/jira/browse/SOLR-7584
> Project: Solr
> Issue Type: Improvement
> Components: SolrJ
> Reporter: Dennis Gove
> Priority: Minor
> Labels: Streaming
> Attachments: SOLR-7584.patch, SOLR-7584.patch, SOLR-7584.patch,
> SOLR-7584.patch
>
>
> Add InnerJoinStream, LeftOuterJoinStream, and supporting classes to the
> Streaming API to allow for joining between sub-streams.
> At its basic, it would look something like this
> {code}
> innerJoin(
> search(collection1, q=*:*, fl="fieldA, fieldB, fieldC", ...),
> search(collection2, q=*:*, fl="fieldA, fieldD, fieldE", ...),
> on="fieldA=fieldA"
> )
> {code}
> or with multi-field on clauses
> {code}
> innerJoin(
> search(collection1, q=*:*, fl="fieldA, fieldB, fieldC", ...),
> search(collection2, q=*:*, fl="fieldA, fieldD, fieldE", ...),
> on="fieldA=fieldA, fieldB=fieldD"
> )
> {code}
> I'd also like to support the option of doing a hash join instead of the
> default merge join but I haven't yet figured out the best way to express
> that. I'd like to let the user tell us which sub-stream should be hashed (the
> least-cost one).
> Also, I've been thinking about field aliasing and might want to add a
> SelectStream which serves the purpose of allowing us to limit the fields
> coming out and rename fields.
> Depends on SOLR-7554
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