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https://issues.apache.org/jira/browse/CRUNCH-57?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13451256#comment-13451256
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Gabriel Reid commented on CRUNCH-57:
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@Rahul, I think you're confusing the use of sorting with min/max calculations.
Min and max aren't calculated using sort, they're calculated by doing an
initial filter in the mappers and then a final filter in a second reducer.
This approach will always be more efficient than sorting the PCollection first.
Outside of the MR context, calculating min/max performance is O(n), and sort is
O(n log n) in the average case -- this translates into resource usage in MR,
although of course things are done in parallel.
> Add a length function to PCollection
> ------------------------------------
>
> Key: CRUNCH-57
> URL: https://issues.apache.org/jira/browse/CRUNCH-57
> Project: Crunch
> Issue Type: New Feature
> Components: Core
> Affects Versions: 0.3.0
> Reporter: Kiyan Ahmadizadeh
> Assignee: Josh Wills
> Attachments: CRUNCH-57.patch
>
>
> Sometimes it's useful and interesting to compute the number of elements in a
> PCollection.
>
> For example, suppose there was an initial PCollection that was then filtered
> into another. If I'm interested in how many elements of the original
> PCollection matched the filter, I'll have to write extra code to compute this.
> PCollections should have a length method that, when called, computes the
> number of elements in the PCollection and returns the result.
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