Spark 1.6: filtering DataFrames generated by describe()

问题

The problem arises when I call describe function on a DataFrame:

val statsDF = myDataFrame.describe()

Calling describe function yields the following output:

statsDF: org.apache.spark.sql.DataFrame = [summary: string, count: string]

I can show statsDF normally by calling statsDF.show()

+-------+------------------+
|summary|             count|
+-------+------------------+
|  count|             53173|
|   mean|104.76128862392568|
| stddev|3577.8184333911513|
|    min|                 1|
|    max|            558407|
+-------+------------------+

I would like now to get the standard deviation and the mean from statsDF, but when I am trying to collect the values by doing something like:

val temp = statsDF.where($"summary" === "stddev").collect()

I am getting Task not serializable exception.

I am also facing the same exception when I call:

statsDF.where($"summary" === "stddev").show()

It looks like we cannot filter DataFrames generated by describe() function?

回答1:

I have considered a toy dataset I had containing some health disease data

val stddev_tobacco = rawData.describe().rdd.map{ 
    case r : Row => (r.getAs[String]("summary"),r.get(1))
}.filter(_._1 == "stddev").map(_._2).collect

回答2:

You can select from the dataframe:

from pyspark.sql.functions import mean, min, max
df.select([mean('uniform'), min('uniform'), max('uniform')]).show()
+------------------+-------------------+------------------+
|      AVG(uniform)|       MIN(uniform)|      MAX(uniform)|
+------------------+-------------------+------------------+
|0.5215336029384192|0.19657711634539565|0.9970412477032209|
+------------------+-------------------+------------------+

You can also register it as a table and query the table:

val t = x.describe()
t.registerTempTable("dt")

%sql 
select * from dt

回答3:

Another option would be to use selectExpr() which also runs optimized, e.g. to obtain the min:

myDataFrame.selectExpr('MIN(count)').head()[0]

回答4:

myDataFrame.describe().filter($"summary"==="stddev").show()

This worked quite nicely on Spark 2.3.0

来源：https://stackoverflow.com/questions/35272086/spark-1-6-filtering-dataframes-generated-by-describe