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https://issues.apache.org/jira/browse/SPARK-24202?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17006214#comment-17006214
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Sam hendley commented on SPARK-24202:
-------------------------------------
I agree that this would be a very valuable change, was there a reason this was
closed without comment?
> Separate SQLContext dependency from SparkSession.implicits
> ----------------------------------------------------------
>
> Key: SPARK-24202
> URL: https://issues.apache.org/jira/browse/SPARK-24202
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.3.0
> Reporter: Gerard Maas
> Priority: Major
> Labels: bulk-closed
>
> The current implementation of the implicits in SparkSession passes the
> current active SQLContext to the SQLImplicits class. This implies that all
> usage of these (extremely helpful) implicits require the prior creation of a
> Spark Session instance.
> Usage is typically done as follows:
>
> {code:java}
> val sparkSession = SparkSession.builder()
> ....getOrCreate()
> import sparkSession.implicits._
> {code}
>
> This is OK in user code, but it burdens the creation of library code that
> uses Spark, where static imports for _Encoder_ support is required.
> A simple example would be:
>
> {code:java}
> class SparkTransformation[In: Encoder, Out: Encoder] {
> def transform(ds: Dataset[In]): Dataset[Out]
> }
> {code}
>
> Attempting to compile such code would result in the following exception:
> {code:java}
> Unable to find encoder for type stored in a Dataset. Primitive types (Int,
> String, etc) and Product types (case classes) are supported by importing
> spark.implicits._ Support for serializing other types will be added in
> future releases.{code}
> The usage of the _SQLContext_ instance in _SQLImplicits_ is limited to two
> utilities to transform _RDD_ and local collections into a _Dataset_.
> These are 2 methods of the 46 implicit conversions offered by this class.
> The request is to separate the two implicit methods that depend on the
> SQLContext instance creation into a separate class:
> {code:java}
> SQLImplicits#214-229
> /**
> * Creates a [[Dataset]] from an RDD.
> *
> * @since 1.6.0
> */
> implicit def rddToDatasetHolder[T : Encoder](rdd: RDD[T]): DatasetHolder[T] =
> {
> DatasetHolder(_sqlContext.createDataset(rdd))
> }
> /**
> * Creates a [[Dataset]] from a local Seq.
> * @since 1.6.0
> */
> implicit def localSeqToDatasetHolder[T : Encoder](s: Seq[T]):
> DatasetHolder[T] = {
> DatasetHolder(_sqlContext.createDataset(s))
> }{code}
> By separating the static methods from these two methods that depend on
> _sqlContext_ into separate classes, we could provide static imports for all
> the other functionality and only require the instance-bound implicits for
> the RDD and collection support (Which is an uncommon use case these days)
> As this is potentially breaking the current interface, this might be a
> candidate for Spark 3.0. Although there's nothing stopping us from creating a
> separate hierarchy for the static encoders already.
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