pyspark one to many join operation

问题

in pyspark dataframe
let say there is dfA and dfB,

dfA : name , class
dfB : class, time

if dfA.select('class').distinct().count() = n, when n is n < 100 , n > 100000,

when I operating the join for this two cases how should I optimize the join?

来源：https://stackoverflow.com/questions/58026274/pyspark-one-to-many-join-operation