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https://issues.apache.org/jira/browse/SPARK-6006?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14336430#comment-14336430
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Apache Spark commented on SPARK-6006:
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User 'saucam' has created a pull request for this issue:
https://github.com/apache/spark/pull/4764
> Optimize count distinct in case of high cardinality columns
> -----------------------------------------------------------
>
> Key: SPARK-6006
> URL: https://issues.apache.org/jira/browse/SPARK-6006
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 1.1.1, 1.2.1
> Reporter: Yash Datta
> Priority: Minor
> Fix For: 1.3.0
>
>
> In case there are a lot of distinct values, count distinct becomes too slow
> since it tries to hash partial results to one map. It can be improved by
> creating buckets/partial maps in an intermediate stage where same key from
> multiple partial maps of first stage hash to the same bucket. Later we can
> sum the size of these buckets to get total distinct count.
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