How does createOrReplaceTempView work in Spark?

Question

I am new to Spark and Spark SQL.

How does createOrReplaceTempView work in Spark?

If we register an RDD of objects as a table will spark keep all the data in memory?

Answer 1

createOrReplaceTempView creates (or replaces if that view name already exists) a lazily evaluated "view" that you can then use like a hive table in Spark SQL. It does not persist to memory unless you cache the dataset that underpins the view.

scala> val s = Seq(1,2,3).toDF("num") s: org.apache.spark.sql.DataFrame = [num: int]  scala> s.createOrReplaceTempView("nums")  scala> spark.table("nums") res22: org.apache.spark.sql.DataFrame = [num: int]  scala> spark.table("nums").cache res23: org.apache.spark.sql.Dataset[org.apache.spark.sql.Row] = [num: int]  scala> spark.table("nums").count res24: Long = 3 

The data is cached fully only after the .count call. Here's proof it's been cached:

Related SO: spark createOrReplaceTempView vs createGlobalTempView

Relevant quote (comparing to persistent table): "Unlike the createOrReplaceTempView command, saveAsTable will materialize the contents of the DataFrame and create a pointer to the data in the Hive metastore." from https://spark.apache.org/docs/latest/sql-programming-guide.html#saving-to-persistent-tables

Note : createOrReplaceTempView was formerly registerTempTable

Answer 2

CreateOrReplaceTempView will create a temporary view of the table on memory it is not persistent at this moment but you can run SQL query on top of that. if you want to save it you can either persist or use saveAsTable to save.

First, we read data in .csv format and then convert to data frame and create a temp view

Reading data in .csv format

val data = spark.read.format("csv").option("header","true").option("inferSchema","true").load("FileStore/tables/pzufk5ib1500654887654/campaign.csv") 

Printing the schema

data.printSchema 

data.createOrReplaceTempView("Data") 

Now we can run SQL queries on top of the table view we just created

  %sql SELECT Week AS Date, Campaign Type, Engagements, Country FROM Data ORDER BY Date ASC