first_value windowing function in pyspark

问题:

I am using pyspark 1.5 getting my data from Hive tables and trying to use windowing functions.

According to this there exists an analytic function called firstValue that will give me the first non-null value for a given window. I know this exists in Hive but I can not find this in pyspark anywhere.

Is there a way to implement this given that pyspark won't allow UserDefinedAggregateFunctions (UDAFs)?

回答1:

Spark >= 2.0:

first takes an optional ignorenulls argument which can mimic the behavior of first_value:

df.select(col("k"), first("v", True).over(w).alias("fv"))

Spark < 2.0:

Available function is called first and can be used as follows:

df = sc.parallelize([
    ("a", None), ("a", 1), ("a", -1), ("b", 3)
]).toDF(["k", "v"])

w = Window().partitionBy("k").orderBy("v")

df.select(col("k"), first("v").over(w).alias("fv"))

but if you want to ignore nulls you'll have to use Hive UDFs directly:

df.registerTempTable("df")

sqlContext.sql("""
    SELECT k, first_value(v, TRUE) OVER (PARTITION BY k ORDER BY v)
    FROM df""")