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https://issues.apache.org/jira/browse/DATAFU-117?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15258239#comment-15258239
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Eyal Allweil commented on DATAFU-117:
-------------------------------------
Ok, I opened a review board for it - can you see it? It's at
https://reviews.apache.org/r/46701/
I think all your previous comments are addressed there, except for the one
about "this.set.add(o) && (this.set.size() == maxAmount". I don't think this
can exceed the max size, because a single add operation can only increment the
set's size by one, and the UDF is executed in a single thread.
I ran a few tests comparing this UDF to a Pig nested foreach with DISTINCT
followed by the builtin COUNT. On small inputs they perform about the same -
even up to a million records - but if you have a situation with more skew (I
checked 10 million records, with about 4 million distincts) then this UDF with
a max value of say, 10000, runs in about four minutes, and the nested
foreach+distinct+count takes more than an hour - probably because it needs to
keep all the distinct records in memory, rather than just reaching the desired
threshold.
> 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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