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https://issues.apache.org/jira/browse/TINKERPOP3-866?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14942444#comment-14942444
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Marko A. Rodriguez commented on TINKERPOP3-866:
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Okay {{group_step_2/}} branch now have {{group.by(key).by(value)}} where if the
{{by(value)}}-traversal has a reducing step on the end of it (e.g. max, min,
fold, count, etc.), then its a reducing group! Easy peasy. The code for
{{GroupStep}} is now much simpler.
The major problem if we merge this. People will have to rewrite their
{{group()}} code :|. What do we think? If the {{dev@}} are happy, I can bring
it up with {{gremin-users@}}.
> GroupStep and Traversal-Based Reductions
> ----------------------------------------
>
> Key: TINKERPOP3-866
> URL: https://issues.apache.org/jira/browse/TINKERPOP3-866
> Project: TinkerPop 3
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.0.1-incubating
> Reporter: Marko A. Rodriguez
> Assignee: Marko A. Rodriguez
> Labels: breaking
> Fix For: 3.1.0-incubating
>
>
> Right now {{GroupStep}} is defined as:
> {code}
> public final class GroupStep<S, K, V, R> extends ReducingBarrierStep<S,
> Map<K, R>> implements MapReducer, TraversalParent {
> private Traversal.Admin<S, K> keyTraversal = null;
> private Traversal.Admin<S, V> valueTraversal = null;
> private Traversal.Admin<Collection<V>, R> reduceTraversal = null;
> ...
> {code}
> Look at {{reduceTraversal}}. It takes a {{Collection<V>}} of "values" and
> reduces them to a "reduction" {{R}}. Why are we using {{Collection<V>}}, why
> is this not:
> {code}
> private Traversal.Admin<V, R> reduceTraversal = null;
> {code}
> Now, when a new {{K}} is created (and reduce is defined), we clone
> {{reduceTraversal}}. Thus, each key has a {{reduceTraversal}} (identical
> clones) that operate in a stream like fashion on {{V}} to yield {{R}}. This
> enables us to remove the {{Collection<V>}} (memory hog) and allows us to
> defined {{GroupCountStep}} in terms of {{GroupStep}} without (?limited?)
> computational cost. HOWEVER, this changes the API as people who did this:
> {code}
> g.V.group.by(label()).by(outE().count()).by(sum(local))
> {code}
> would now have to do this:
> {code}
> g.V.group.by(label()).by(outE().count()).by(sum())
> {code}
> Its very minor, given the speed up we would gain and the ability for us to
> now do "groupCount" efficiently on arbitrary values -- not just bulks (e.g.
> sacks).
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