如何创建地图数据集? [英] How to create a Dataset of Maps?
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问题描述
我正在使用Spark 2.2,尝试在Map
的Seq
上调用spark.createDataset
时遇到麻烦.
I'm using Spark 2.2 and am running into troubles when attempting to call spark.createDataset
on a Seq
of Map
.
Spark Shell会话中的代码和输出如下:
Code and output from my Spark Shell session follow:
// createDataSet on Seq[T] where T = Int works
scala> spark.createDataset(Seq(1, 2, 3)).collect
res0: Array[Int] = Array(1, 2, 3)
scala> spark.createDataset(Seq(Map(1 -> 2))).collect
<console>:24: error: 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.
spark.createDataset(Seq(Map(1 -> 2))).collect
^
// createDataSet on a custom case class containing Map works
scala> case class MapHolder(m: Map[Int, Int])
defined class MapHolder
scala> spark.createDataset(Seq(MapHolder(Map(1 -> 2)))).collect
res2: Array[MapHolder] = Array(MapHolder(Map(1 -> 2)))
我已经尝试过import spark.implicits._
,尽管我相当确定它是由Spark shell会话隐式导入的.
I've tried import spark.implicits._
, though I'm fairly certain that's implicitly imported by the Spark shell session.
这是当前编码器无法解决的情况吗?
Is this is a case not covered by current encoders?
推荐答案
2.2中未涉及它,但可以轻松解决.您可以使用ExpressionEncoder
显式添加所需的Encoder
:
It is not covered in 2.2, but can be easily addressed. You can add required Encoder
using ExpressionEncoder
, either explicitly:
import org.apache.spark.sql.catalyst.encoders.ExpressionEncoder
import org.apache.spark.sql.Encoder
spark
.createDataset(Seq(Map(1 -> 2)))(ExpressionEncoder(): Encoder[Map[Int, Int]])
或implicitly
:
implicit def mapIntIntEncoder: Encoder[Map[Int, Int]] = ExpressionEncoder()
spark.createDataset(Seq(Map(1 -> 2)))
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