Dataset — Strongly-Typed Structured Query with Encoder

Dataset is a strongly-typed data structure in Spark SQL that represents a structured query with encoders.

spark sql Dataset.png
Figure 1. Dataset’s Internals
Note
Given the picture above, one could say that a Dataset is a pair of an Encoder and QueryExecution (that in turn is a LogicalPlan in a SparkSession)

Datasets are lazy and structured query expressions are only triggered when an action is invoked. Internally, a Dataset represents a logical plan that describes the computation query required to produce the data (in a given session).

A Dataset is a result of executing a query expression against data storage like files, Hive tables or JDBC databases. The structured query expression can be described by a SQL query, a Column-based SQL expression or a Scala/Java lambda function. And that is why Dataset operations are available in three variants.

import org.apache.spark.sql.SparkSession
val spark: SparkSession = ...

scala> val dataset = spark.range(5)
dataset: org.apache.spark.sql.Dataset[Long] = [id: bigint]

// Variant 1: filter operator accepts a Scala function
dataset.filter(n => n % 2 == 0).count

// Variant 2: filter operator accepts a Column-based SQL expression
dataset.filter('value % 2 === 0).count

// Variant 3: filter operator accepts a SQL query
dataset.filter("value % 2 = 0").count

The Dataset API offers declarative and type-safe operators that makes for an improved experience for data processing (comparing to DataFrames that were a set of index- or column name-based Rows).

Note

Dataset was first introduced in Apache Spark 1.6.0 as an experimental feature, and has since turned itself into a fully supported API.

As of Spark 2.0.0, DataFrame - the flagship data abstraction of previous versions of Spark SQL - is currently a mere type alias for Dataset[Row]:

type DataFrame = Dataset[Row]

Dataset offers convenience of RDDs with the performance optimizations of DataFrames and the strong static type-safety of Scala. The last feature of bringing the strong type-safety to DataFrame makes Dataset so appealing. All the features together give you a more functional programming interface to work with structured data.

scala> spark.range(1).filter('id === 0).explain(true)
== Parsed Logical Plan ==
'Filter ('id = 0)
+- Range (0, 1, splits=8)

== Analyzed Logical Plan ==
id: bigint
Filter (id#51L = cast(0 as bigint))
+- Range (0, 1, splits=8)

== Optimized Logical Plan ==
Filter (id#51L = 0)
+- Range (0, 1, splits=8)

== Physical Plan ==
*Filter (id#51L = 0)
+- *Range (0, 1, splits=8)

scala> spark.range(1).filter(_ == 0).explain(true)
== Parsed Logical Plan ==
'TypedFilter <function1>, class java.lang.Long, [StructField(value,LongType,true)], unresolveddeserializer(newInstance(class java.lang.Long))
+- Range (0, 1, splits=8)

== Analyzed Logical Plan ==
id: bigint
TypedFilter <function1>, class java.lang.Long, [StructField(value,LongType,true)], newInstance(class java.lang.Long)
+- Range (0, 1, splits=8)

== Optimized Logical Plan ==
TypedFilter <function1>, class java.lang.Long, [StructField(value,LongType,true)], newInstance(class java.lang.Long)
+- Range (0, 1, splits=8)

== Physical Plan ==
*Filter <function1>.apply
+- *Range (0, 1, splits=8)

It is only with Datasets to have syntax and analysis checks at compile time (that was not possible using DataFrame, regular SQL queries or even RDDs).

Using Dataset objects turns DataFrames of Row instances into a DataFrames of case classes with proper names and types (following their equivalents in the case classes). Instead of using indices to access respective fields in a DataFrame and cast it to a type, all this is automatically handled by Datasets and checked by the Scala compiler.

Datasets use Catalyst Query Optimizer and Tungsten to optimize query performance.

A Dataset object requires a SparkSession, a QueryExecution plan, and an Encoder (for fast serialization to and deserialization from InternalRow).

If however a LogicalPlan is used to create a Dataset, the logical plan is first executed (using the current SessionState in the SparkSession) that yields the QueryExecution plan.

A Dataset is Queryable and Serializable, i.e. can be saved to a persistent storage.

Note
SparkSession and QueryExecution are transient attributes of a Dataset and therefore do not participate in Dataset serialization. The only firmly-tied feature of a Dataset is the Encoder.

You can convert a type-safe Dataset to a "untyped" DataFrame or access the RDD that is generated after executing the query. It is supposed to give you a more pleasant experience while transitioning from the legacy RDD-based or DataFrame-based APIs you may have used in the earlier versions of Spark SQL or encourage migrating from Spark Core’s RDD API to Spark SQL’s Dataset API.

The default storage level for Datasets is MEMORY_AND_DISK because recomputing the in-memory columnar representation of the underlying table is expensive. You can however persist a Dataset.

Note
Spark 2.0 has introduced a new query model called Structured Streaming for continuous incremental execution of structured queries. That made possible to consider Datasets a static and bounded as well as streaming and unbounded data sets with a single unified API for different execution models.

A Dataset is local if it was created from local collections using SparkSession.emptyDataset or SparkSession.createDataset methods and their derivatives like toDF. If so, the queries on the Dataset can be optimized and run locally, i.e. without using Spark executors.

Note
Dataset makes sure that the underlying QueryExecution is analyzed and checked.
Table 1. Dataset’s Properties
Name Description

boundEnc

ExpressionEncoder

Used when…​FIXME

exprEnc

Implicit ExpressionEncoder

Used when…​FIXME

logicalPlan

Logical plan

rdd

(lazily-created) RDD of JVM objects of type T (as converted from rows in Dataset in the internal binary row format).

rdd: RDD[T]
Note
rdd gives RDD with the extra execution step to convert rows from their internal binary row format to JVM objects that will impact the JVM memory as the objects are inside JVM (while were outside before). You should not use rdd directly.

Internally, rdd first creates a new logical plan that deserializes the Dataset’s logical plan.

val dataset = spark.range(5).withColumn("group", 'id % 2)
scala> dataset.rdd.toDebugString
res1: String =
(8) MapPartitionsRDD[8] at rdd at <console>:26 [] // <-- extra deserialization step

MapPartitionsRDD[7] at rdd at <console>:26 []

MapPartitionsRDD[6] at rdd at <console>:26 []

MapPartitionsRDD[5] at rdd at <console>:26 []

ParallelCollectionRDD[4] at rdd at <console>:26 []

scala> dataset.queryExecution.toRdd.toDebugString res2: String = (8) MapPartitionsRDD[11] at toRdd at <console>:26 []

MapPartitionsRDD[10] at toRdd at <console>:26 []

ParallelCollectionRDD[9] at toRdd at <console>:26 [] ----

rdd then requests SessionState to execute the logical plan to get the corresponding RDD of binary rows.

NOTE: rdd uses SparkSession to access SessionState.

rdd then requests the Dataset’s ExpressionEncoder for the data type of the rows (using deserializer expression) and maps over them (per partition) to create records of the expected type T.

NOTE: rdd is at the "boundary" between the internal binary row format and the JVM type of the dataset. Avoid the extra deserialization step to lower JVM memory requirements of your Spark application.

sqlContext

Lazily-created SQLContext

Used when…​FIXME

resolve Internal Method

resolve(colName: String): NamedExpression
Caution
FIXME

Creating Dataset Instance

Dataset takes the following when created:

Note
You can also create a Dataset using LogicalPlan that is immediately executed using SessionState.

Internally, Dataset requests QueryExecution to analyze itself.

Dataset initializes the internal registries and counters.

Is Dataset Local? — isLocal Method

isLocal: Boolean

isLocal flag is enabled (i.e. true) when operators like collect or take could be run locally, i.e. without using executors.

Internally, isLocal checks whether the logical query plan of a Dataset is LocalRelation.

Is Dataset Streaming? — isStreaming method

isStreaming: Boolean

isStreaming is enabled (i.e. true) when the logical plan is streaming.

Internally, isStreaming takes the Dataset’s logical plan and gives whether the plan is streaming or not.

Implicit Type Conversions to Datasets — toDS and toDF methods

DatasetHolder case class offers three methods that do the conversions from Seq[T] or RDD[T] types to a Dataset[T]:

  • toDS(): Dataset[T]

  • toDF(): DataFrame

  • toDF(colNames: String*): DataFrame

Note
DataFrame is a mere type alias for Dataset[Row] since Spark 2.0.0.

DatasetHolder is used by SQLImplicits that is available to use after importing implicits object of SparkSession.

val spark: SparkSession = ...
import spark.implicits._

scala> val ds = Seq("I am a shiny Dataset!").toDS
ds: org.apache.spark.sql.Dataset[String] = [value: string]

scala> val df = Seq("I am an old grumpy DataFrame!").toDF
df: org.apache.spark.sql.DataFrame = [value: string]

scala> val df = Seq("I am an old grumpy DataFrame!").toDF("text")
df: org.apache.spark.sql.DataFrame = [text: string]

scala> val ds = sc.parallelize(Seq("hello")).toDS
ds: org.apache.spark.sql.Dataset[String] = [value: string]
Note

This import of implicits object’s values is automatically executed in Spark Shell and so you don’t need to do anything but use the conversions.

scala> spark.version
res11: String = 2.0.0

scala> :imports
 1) import spark.implicits._  (59 terms, 38 are implicit)
 2) import spark.sql          (1 terms)
val spark: SparkSession = ...
import spark.implicits._

case class Token(name: String, productId: Int, score: Double)
val data = Seq(
  Token("aaa", 100, 0.12),
  Token("aaa", 200, 0.29),
  Token("bbb", 200, 0.53),
  Token("bbb", 300, 0.42))

// Transform data to a Dataset[Token]
// It doesn't work with type annotation
// https://issues.apache.org/jira/browse/SPARK-13456
val ds = data.toDS

// ds: org.apache.spark.sql.Dataset[Token] = [name: string, productId: int ... 1 more field]

// Transform data into a DataFrame with no explicit schema
val df = data.toDF

// Transform DataFrame into a Dataset
val ds = df.as[Token]

scala> ds.show
+----+---------+-----+
|name|productId|score|
+----+---------+-----+
| aaa|      100| 0.12|
| aaa|      200| 0.29|
| bbb|      200| 0.53|
| bbb|      300| 0.42|
+----+---------+-----+

scala> ds.printSchema
root
 |-- name: string (nullable = true)
 |-- productId: integer (nullable = false)
 |-- score: double (nullable = false)

// In DataFrames we work with Row instances
scala> df.map(_.getClass.getName).show(false)
+--------------------------------------------------------------+
|value                                                         |
+--------------------------------------------------------------+
|org.apache.spark.sql.catalyst.expressions.GenericRowWithSchema|
|org.apache.spark.sql.catalyst.expressions.GenericRowWithSchema|
|org.apache.spark.sql.catalyst.expressions.GenericRowWithSchema|
|org.apache.spark.sql.catalyst.expressions.GenericRowWithSchema|
+--------------------------------------------------------------+

// In Datasets we work with case class instances
scala> ds.map(_.getClass.getName).show(false)
+---------------------------+
|value                      |
+---------------------------+
|$line40.$read$$iw$$iw$Token|
|$line40.$read$$iw$$iw$Token|
|$line40.$read$$iw$$iw$Token|
|$line40.$read$$iw$$iw$Token|
+---------------------------+

Internals of toDS

Internally, the Scala compiler makes toDS implicitly available to any Seq[T] (using SQLImplicits.localSeqToDatasetHolder implicit method).

Note
This and other implicit methods are in scope whenever you do import spark.implicits._.

The input Seq[T] is converted into Dataset[T] by means of SQLContext.createDataset that in turn passes all calls on to SparkSession.createDataset. Once created, the Dataset[T] is wrapped in DatasetHolder[T] with toDS that just returns the input ds.

Queryable

Caution
FIXME

Tracking Multi-Job SQL Query Executions — withNewExecutionId Internal Method

withNewExecutionId[U](body: => U): U

withNewExecutionId is a private[sql] operator that executes the input body action using SQLExecution.withNewExecutionId that sets the execution id local property set.

Note
withNewExecutionId is used in foreach, foreachPartition, and (private) collect.

Creating DataFrame — ofRows Internal Method

ofRows(sparkSession: SparkSession, logicalPlan: LogicalPlan): DataFrame
Note
ofRows is a private[sql] operator that can only be accessed from code in org.apache.spark.sql package. It is not a part of Dataset's public API.

ofRows returns DataFrame (which is the type alias for Dataset[Row]). ofRows uses RowEncoder to convert the schema (based on the input logicalPlan logical plan).

Internally, ofRows prepares the input logicalPlan for execution and creates a Dataset[Row] with the current SparkSession, the QueryExecution and RowEncoder.

Further reading or watching

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