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https://issues.apache.org/jira/browse/DRILL-20?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13563074#comment-13563074
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Ted Dunning commented on DRILL-20:
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The BigQuery guys point out that the intent of limit is often really just TOP
and that it is useful to provide a TOP aggregator that is clearly marked as
approximate rather than requiring a fairly subtle optimization of sort + limit.
The same applies for count(distinct).
For both of these, the biggest and strongest argument in my mind is the fact
that approximate algorithms are incredibly efficient but it really is nice if
sort+limit or count(distinct) produce exact results. TOP and UNIQUES provide a
very useful way for users to indicate that approximate results are acceptable.
> Limit Operator Reference Implementation
> ---------------------------------------
>
> Key: DRILL-20
> URL: https://issues.apache.org/jira/browse/DRILL-20
> Project: Apache Drill
> Issue Type: Improvement
> Reporter: Chris Merrick
> Attachments: limit-reference.patch
>
>
> Build off of Jacques work on reference implementations - the limit operator.
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