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https://issues.apache.org/jira/browse/DATAFU-117?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15236029#comment-15236029
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Matthew Hayes commented on DATAFU-117:
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Oh something else I thought about that would be useful is to compare the
performance of this UDF to simply using DISTINCT. What I usually do is compare
total_mappers * avg_map_time + total_reducers * avg_reduce_time. It would be
good to use this information to document when this will perform better than
DISTINCT.
> New UDF - CountDistinctUpTo
> ---------------------------
>
> Key: DATAFU-117
> URL: https://issues.apache.org/jira/browse/DATAFU-117
> Project: DataFu
> Issue Type: New Feature
> Reporter: Eyal Allweil
> Attachments: DATAFU-117.patch
>
>
> A UDF that counts distinct tuples within a bag, but only up to a preset
> limit. If the bag contains more distinct tuples than the limit, the UDF
> returns the limit.
> This UDF can run reasonably well even on large bags if the limit chosen is
> small enough though the count is done in memory.
> We use this UDF in PayPal for filtering, when we don't need to use the actual
> tuples afterward.
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