Spark中groupByKey、reduceByKey與sortByKey

摘要： groupByKey 把相同的key的資料分組到一個集合序列當中： [("hello",1), ("world",1), ("hello",1), ("fly",1), ("hello"...

groupByKey 把相同的key的資料分組到一個集合序列當中：

[("hello",1), ("world",1), ("hello",1), ("fly",1), ("hello",1), ("world",1)] --> [("hello",(1,1,1)),("word",(1,1)),("fly",(1))]

reduceByKey把相同的key的資料聚合到一起並進行相應的計算：

[("hello",1), ("world",1), ("hello",1), ("fly",1), ("hello",1), ("world",1)]  add--> [("hello",3）,("word",2),("fly",1)]

sortByKey按key的大小排序，預設為升序排序：

[(3,"hello"）,(2,"word"),(1,"fly")]  -->   [(1,"fly")，(2,"word")，(3,"hello")] 

groupByKey、reduceByKey及sortByKey的比較：

from pyspark import SparkConf, SparkContext
from operator import add

conf = SparkConf()
sc = SparkContext(conf=conf)


def func_by_key():
data = [
"hello world", "hello fly", "hello world",
"hello fly", "hello fly", "hello fly"
]
data_rdd = sc.parallelize(data)
word_rdd = data_rdd.flatMap(lambda s: s.split(" ")).map(lambda x: (x, 1))
group_by_key_rdd = word_rdd.groupByKey()
print("groupByKey:{}".format(group_by_key_rdd.mapValues(list).collect()))
print("groupByKey mapValues(len):{}".format(
group_by_key_rdd.mapValues(len).collect()
))

reduce_by_key_rdd = word_rdd.reduceByKey(add)
print("reduceByKey:{}".format(reduce_by_key_rdd.collect()))

print("sortByKey:{}".format(reduce_by_key_rdd.map(
lambda x: (x[1], x[0])
).sortByKey().map(lambda x: (x[0], x[1])).collect()))

func_by_key()
sc.stop()

"""
result：
groupByKey:[('fly', [1, 1, 1, 1]), ('world', [1, 1]), ('hello', [1, 1, 1, 1, 1, 1])]
groupByKey mapValues(len):[('fly', 4), ('world', 2), ('hello', 6)]
reduceByKey:[('fly', 4), ('world', 2), ('hello', 6)]
sortByKey:[(2, 'world'), (4, 'fly'), (6, 'hello')]
"""

從結果可以看出，groupByKey對分組後的每個key的value做mapValues(len)後的結果與reduceByKey的結果一致，即：如果分組後要對每一個key所對應的值進行操作則應直接用 reduceByKey；sortByKey是按key排序，如果要對value排序，可以交換key與value的位置，再排序。