[jira] [Created] (SPARK-28456) Add a public API `Encoder.copyEncoder` to allow creating Encoder without touching Scala reflections

["Shixiong Zhu (JIRA)" <ji...@apache.org>]

Shixiong Zhu created SPARK-28456:
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             Summary: Add a public API `Encoder.copyEncoder` to allow creating Encoder without touching Scala reflections
                 Key: SPARK-28456
                 URL: https://issues.apache.org/jira/browse/SPARK-28456
             Project: Spark
          Issue Type: New Feature
          Components: SQL
    Affects Versions: 2.4.3
            Reporter: Shixiong Zhu
            Assignee: Shixiong Zhu


Because `Encoder` is not thread safe, the user cannot reuse an `Encoder` in multiple `Dataset`s. However, creating an `Encoder` for a complicated class is slow due to Scala reflections. To reduce the cost of Encoder creation, right now I usually use the private API `ExpressionEncoder.copy` as follows:

{code}
object FooEncoder {
 private lazy val _encoder: ExpressionEncoder[Foo] = ExpressionEncoder[Foo]()
 implicit def encoder: ExpressionEncoder[Foo] = _encoder.copy()
}
{code}

This PR proposes a new method `copyEncoder` in `Encoder` so that the above codes can be rewritten using public APIs.

{code}
object FooEncoder {
 private lazy val _encoder: Encoder[Foo] = Encoders.product[Foo]()
 implicit def encoder: Encoder[Foo] = _encoder.copyEncoder()
}
{code}

Regarding the method name, 
- Why not use `copy`? It conflicts with `case class`'s copy.
- Why not use `clone`? It conflicts with `Object.clone`.



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