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https://issues.apache.org/jira/browse/FLINK-1664?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14355054#comment-14355054
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Maximilian Michels commented on FLINK-1664:
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+1
> Forbid sorting on POJOs
> -----------------------
>
> Key: FLINK-1664
> URL: https://issues.apache.org/jira/browse/FLINK-1664
> Project: Flink
> Issue Type: Bug
> Components: JobManager
> Affects Versions: 0.8.0, 0.9
> Reporter: Fabian Hueske
> Assignee: Fabian Hueske
>
> Flink's groupSort, partitionSort, and outputSort operators allow to sort
> partitions or groups of a DataSet.
> If the sort is defined on a POJO field, the sort order is not well defined.
> Internally, the POJO is recursively decomposed into atomic fields (primitives
> or generic types) and sorted by sorting these atomic fields. Thereby, the
> order of these atomic fields is not well defined (I believe it is
> lexicographic order of the POJO's member names).
> IMO, the best approach is to forbid sorting on POJO types for now. Instead,
> it is always possible to select the nested fields of the POJO that should be
> used for sorting. Later we can relax this restriction.
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