scala - What is going wrong with `unionAll` of Spark `DataFrame`?

Question

Using Spark 1.5.0 and given the following code, I expect unionAll to union DataFrames based on their column name. In the code, I'm using some FunSuite for passing in SparkContext sc:

object Entities {

  case class A (a: Int, b: Int)
  case class B (b: Int, a: Int)

  val as = Seq(
    A(1,3),
    A(2,4)
  )

  val bs = Seq(
    B(5,3),
    B(6,4)
  )
}

class UnsortedTestSuite extends SparkFunSuite {

  configuredUnitTest("The truth test.") { sc =>
    val sqlContext = new SQLContext(sc)
    import sqlContext.implicits._
    val aDF = sc.parallelize(Entities.as, 4).toDF
    val bDF = sc.parallelize(Entities.bs, 4).toDF
    aDF.show()
    bDF.show()
    aDF.unionAll(bDF).show
  }
}

Output:

+---+---+
|  a|  b|
+---+---+
|  1|  3|
|  2|  4|
+---+---+

+---+---+
|  b|  a|
+---+---+
|  5|  3|
|  6|  4|
+---+---+

+---+---+
|  a|  b|
+---+---+
|  1|  3|
|  2|  4|
|  5|  3|
|  6|  4|
+---+---+

Why does the result contain intermixed "b" and "a" columns, instead of aligning columns bases on column names? Sounds like a serious bug!?

Answer 1

It doesn't look like a bug at all. What you see is a standard SQL behavior and every major RDMBS, including PostgreSQL, MySQL, Oracle and MS SQL behaves exactly the same. You'll find SQL Fiddle examples linked with names.

To quote PostgreSQL manual:

In order to calculate the union, intersection, or difference of two queries, the two queries must be "union compatible", which means that they return the same number of columns and the corresponding columns have compatible data types

Column names, excluding the first table in the set operation, are simply ignored.

This behavior comes directly form the Relational Algebra where basic building block is a tuple. Since tuples are ordered an union of two sets of tuples is equivalent (ignoring duplicates handling) to the output you get here.

If you want to match using names you can do something like this

import org.apache.spark.sql.DataFrame
import org.apache.spark.sql.functions.col

def unionByName(a: DataFrame, b: DataFrame): DataFrame = {
  val columns = a.columns.toSet.intersect(b.columns.toSet).map(col).toSeq
  a.select(columns: _*).unionAll(b.select(columns: _*))
}

To check both names and types it is should be enough to replace columns with:

a.dtypes.toSet.intersect(b.dtypes.toSet).map{case (c, _) => col(c)}.toSeq