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Posted to issues@spark.apache.org by "Yash Datta (JIRA)" <ji...@apache.org> on 2015/02/25 13:21:04 UTC

[jira] [Created] (SPARK-6006) Optimize count distinct in case high cardinality columns

Yash Datta created SPARK-6006:
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             Summary: Optimize count distinct in case high cardinality columns
                 Key: SPARK-6006
                 URL: https://issues.apache.org/jira/browse/SPARK-6006
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 1.2.1, 1.1.1
            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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