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https://issues.apache.org/jira/browse/SPARK-4963?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14259977#comment-14259977
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Apache Spark commented on SPARK-4963:
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User 'yanbohappy' has created a pull request for this issue:
https://github.com/apache/spark/pull/3827
> SchemaRDD.sample may return wrong results
> -----------------------------------------
>
> Key: SPARK-4963
> URL: https://issues.apache.org/jira/browse/SPARK-4963
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 1.2.0
> Reporter: Cheng Lian
>
> This {{sbt/sbt hive/console}} session can easily reproduce this issue:
> {code}
> sql("SELECT * FROM src WHERE key % 2 = 0").
> sample(withReplacement = false, fraction = 0.05).
> registerTempTable("sampled")
> println(table("sampled").queryExecution)
> val query = sql("SELECT * FROM sampled WHERE key % 2 = 1")
> println(query.queryExecution)
> // Should print `true'
> println((1 to 10).map(_ => query.collect().isEmpty).reduce(_ && _))
> {code}
> Notice that when fraction is less than 0.4, {{GapSamplingIterator}} is used
> to do the sampling. My guess is that there’s something to do with the
> underlying mutable row objects used in {{HiveTableScan}}, but haven't figured
> out the root cause.
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