[jira] [Created] (SPARK-26696) Dataset encoder should be publicly accessible

["Simeon Simeonov (JIRA)" <ji...@apache.org>]

Simeon Simeonov created SPARK-26696:
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             Summary: Dataset encoder should be publicly accessible
                 Key: SPARK-26696
                 URL: https://issues.apache.org/jira/browse/SPARK-26696
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 2.4.0
            Reporter: Simeon Simeonov


As a platform, Spark should enable framework developers to accomplish outside of the Spark codebase much of what can be accomplished inside the Spark codebase. One of the obstacles to this is a historical pattern of excessive data hiding in Spark, e.g., {{expr}} in {{Column}} not being accessible. This issue is an example of this pattern when it comes to {{Dataset}}.

Consider a transformation with the signature `def foo[A](ds: Dataset[A]): Dataset[A]`, which requires the use of {{toDF()}}. To get back to {{Dataset[A]}} would require calling {{.as[A]}}, which requires an implicit {{Encoder[A]}}. A naive approach would change the function signature to `foo[A : Encoder]` but this is poor API design that requires unnecessarily carrying of implicits from user code into framework code. We know `Encoder[A]` exists because we have access to an instance of `Dataset[A]`... but its `encoder` is not accessible.

The solution is simple: make {{encoder}} a {{@transient val}} just as is the case with {{queryExecution}}.



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