Spark 2.0 Dataset vs DataFrame

Question

starting out with spark 2.0.1 I got some questions. I read a lot of documentation but so far could not find sufficient answers:

• What is the difference between
  • df.select("foo")
  • df.select($"foo")
• do I understand correctly that
  • myDataSet.map(foo.someVal) is typesafe and will not convert into RDD but stay in DataSet representation / no additional overhead (performance wise for 2.0.0)
• all the other commands e.g. select, .. are just syntactic sugar. They are not typesafe and a map could be used instead. How could I df.select("foo") type-safe without a map statement?
  • why should I use a UDF / UADF instead of a map (assuming map stays in the dataset representation)?

Answer 1

1. Difference between df.select("foo") and df.select($"foo") is signature. The former one takes at least one String, the later one zero or more Columns. There is no practical difference beyond that.

2. myDataSet.map(foo.someVal) type checks, but as any Dataset operation uses RDD of objects, and compared to DataFrame operations, there is a significant overhead. Let's take a look at a simple example:

   case class FooBar(foo: Int, bar: String) val ds = Seq(FooBar(1, "x")).toDS ds.map(_.foo).explain 

   == Physical Plan == *SerializeFromObject [input[0, int, true] AS value#123] +- *MapElements <function1>, obj#122: int    +- *DeserializeToObject newInstance(class $line67.$read$$iw$$iw$FooBar), obj#121: $line67.$read$$iw$$iw$FooBar       +- LocalTableScan [foo#117, bar#118] 

   As you can see this execution plan requires access to all fields and has to DeserializeToObject.

3. No. In general other methods are not syntactic sugar and generate a significantly different execution plan. For example:

   ds.select($"foo").explain 

   == Physical Plan == LocalTableScan [foo#117] 

   Compared to the plan shown before it can access column directly. It is not so much a limitation of the API but a result of a difference in the operational semantics.

4. How could I df.select("foo") type-safe without a map statement?

   There is no such option. While typed columns allow you to transform statically Dataset into another statically typed Dataset:

   ds.select($"bar".as[Int]) 

   there are not type safe. There some other attempts to include type safe optimized operations, like typed aggregations, but this experimental API.

5. why should I use a UDF / UADF instead of a map

   It is completely up to you. Each distributed data structure in Spark provides its own advantages and disadvantages (see for example Spark UDAF with ArrayType as bufferSchema performance issues).

Personally, I find statically typed Dataset to be the least useful:

• Don't provide the same range of optimizations as Dataset[Row] (although they share storage format and some execution plan optimizations it doesn't fully benefit from code generation or off-heap storage) nor access to all the analytical capabilities of the DataFrame.

• Typed transformations are black boxes, and effectively create analysis barrier for the optimizer. For example selections (filters) cannot be be pushed over typed transformation:

  ds.groupBy("foo").agg(sum($"bar") as "bar").as[FooBar].filter(x => true).where($"foo" === 1).explain 

  == Physical Plan == *Filter (foo#133 = 1) +- *Filter <function1>.apply    +- *HashAggregate(keys=[foo#133], functions=[sum(cast(bar#134 as double))])       +- Exchange hashpartitioning(foo#133, 200)          +- *HashAggregate(keys=[foo#133], functions=[partial_sum(cast(bar#134 as double))])             +- LocalTableScan [foo#133, bar#134] 

  Compared to:

  ds.groupBy("foo").agg(sum($"bar") as "bar").as[FooBar].where($"foo" === 1).explain 

  == Physical Plan == *HashAggregate(keys=[foo#133], functions=[sum(cast(bar#134 as double))]) +- Exchange hashpartitioning(foo#133, 200)    +- *HashAggregate(keys=[foo#133], functions=[partial_sum(cast(bar#134 as double))])       +- *Filter (foo#133 = 1)          +- LocalTableScan [foo#133, bar#134]  

  This impacts features like predicate pushdown or projection pushdown.

• There are not as flexible as RDDs with only a small subset of types supported natively.

• "Type safety" with Encoders is disputable when Dataset is converted using as method. Because data shape is not encoded using a signature, a compiler can only verify the existence of an Encoder.

Related questions:

• Perform a typed join in Scala with Spark Datasets
• Spark 2.0 DataSets groupByKey and divide operation and type safety