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https://issues.apache.org/jira/browse/SOLR-8925?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15232800#comment-15232800
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Dennis Gove commented on SOLR-8925:
-----------------------------------
The order in the walk parameter might be confusing.
{code}
walk ="author->user”,
{code}
In other expressions where we're checking equality between two streams we use a
standard of firstStreamField = secondStreamField. In gatherNodes, the field on
the right appears to go with the first stream while the field on the left goes
with the second stream. I'm not suggesting I don't like the author->user
structure, because I do, but perhaps that the use of collection as the first
param might lead to confusion.
> Add gatherNodes Streaming Expression to support breadth first traversals
> ------------------------------------------------------------------------
>
> Key: SOLR-8925
> URL: https://issues.apache.org/jira/browse/SOLR-8925
> Project: Solr
> Issue Type: New Feature
> Reporter: Joel Bernstein
> Assignee: Joel Bernstein
> Fix For: 6.1
>
>
> The gatherNodes Streaming Expression is a flexible general purpose breadth
> first graph traversal. It uses the same parallel join under the covers as
> (SOLR-8888) but is much more generalized and can be used for a wide range of
> use cases.
> Sample syntax:
> {code}
> gatherNodes(
> friends,
> gatherNodes(
> friends,
> search(articles, q=“body:(query
> 1)”, fl=“author”),
> walk ="author->user”,
> gather="friend"),
> walk=“friend-> user”,
> gather="friend",
> scatter=“roots, branches, leaves”
> )
> {code}
> The expression above is evaluated as follows:
> 1) The inner search() expression is evaluated on the *articles* collection,
> emitting a Stream of Tuples with the author field populated.
> 2) The inner gatherNodes() expression reads the Tuples form the search()
> stream and traverses to the *friends* collection by performing a distributed
> join between articles.author and friends.user field. It gathers the value
> from the *friend* field during the join.
> 3) The inner gatherNodes() expression then emits the *friend* Tuples. By
> default the gatherNodes function emits only the leaves which in this case are
> the *friend* tuples.
> 4) The outer gatherNodes() expression reads the *friend* Tuples and Traverses
> again in the "friends" collection, this time performing the join between
> *friend* Tuples emitted in step 3. This collects the friend of friends.
> 5) The outer gatherNodes() expression emits the entire graph that was
> collected. This is controlled by the "scatter" parameter. In the example the
> *root* nodes are the authors, the *branches* are the author's friends and the
> *leaves* are the friend of friends.
> This traversal is fully distributed and cross collection.
> Like all streaming expressions the gather nodes expression can be combined
> with other streaming expressions. For example the following expression uses a
> hashJoin to intersect the network of friends rooted to authors found with
> different queries:
> {code}
> hashInnerJoin(
> gatherNodes(friends,
> gatherNodes(friends
> search(articles,
> q=“body:(queryA)”, fl=“author”),
> walk ="author->user”,
> gather="friend"),
> walk=“friend -> user”,
> gather="friend",
> scatter=“branches, leaves”),
> gatherNodes(friends,
> gatherNodes(friends
> search(articles,
> q=“body:(queryB)”, fl=“author”),
> walk ="author->user”,
> gather="friend"),
> walk=“friend -> user”,
> gather="friend",
> scatter=“branches, leaves”),
> on=“friend”
> )
> {code}
>
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