apache-spark Introduction to Apache Spark DataFrames Spark Dataframe explained


Example

In Spark, a DataFrame is a distributed collection of data organized into named columns. It is conceptually equivalent to a table in a relational database or a data frame in R/Python, but with richer optimizations under the hood. DataFrames can be constructed from a wide array of sources such as structured data files, tables in Hive, external databases, or existing RDDs.

Ways of creating Dataframe

val data= spark.read.json("path to json")

val df = spark.read.format("com.databricks.spark.csv").load("test.txt") in the options field, you can provide header, delimiter, charset and much more

you can also create Dataframe from an RDD

val rdd = sc.parallelize(
  Seq(
    ("first", Array(2.0, 1.0, 2.1, 5.4)),
    ("test", Array(1.5, 0.5, 0.9, 3.7)),
    ("choose", Array(8.0, 2.9, 9.1, 2.5))
  )
)

val dfWithoutSchema = spark.createDataFrame(rdd)

If you want to create df with schema

def createDataFrame(rowRDD: RDD[Row], schema: StructType): DataFrame

Why we need Dataframe if Spark has provided RDD

An RDD is merely a Resilient Distributed Dataset that is more of a blackbox of data that cannot be optimized as the operations that can be performed against it, are not as constrained.

No inbuilt optimization engine: When working with structured data, RDDs cannot take advantages of Spark’s advanced optimizers including catalyst optimizer and Tungsten execution engine. Developers need to optimize each RDD based on its attributes. Handling structured data: Unlike Dataframe and datasets, RDDs don’t infer the schema of the ingested data and requires the user to specify it.

DataFrames in Spark have their execution automatically optimized by a query optimizer. Before any computation on a DataFrame starts, the Catalyst optimizer compiles the operations that were used to build the DataFrame into a physical plan for execution. Because the optimizer understands the semantics of operations and structure of the data, it can make intelligent decisions to speed up computation.

Limitation of DataFrame

Compile-time type safety: Dataframe API does not support compile time safety which limits you from manipulating data when the structure is not known.