Function to join Spark DataFrames using Spark context to improve performance.

UnionSparkDataFrame.scala

  def unionAll(dfs: Seq[DataFrame]): DataFrame = {
    val spark = SparkSession.builder().getOrCreate()
    spark.sqlContext.createDataFrame(
      spark.sparkContext.union(dfs.map(df => df.rdd)),
      dfs.head.schema
    )
  }