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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