reduceByKey(func) converts a dataset of (K, V) pairs, into a dataset of (K, V) pairs where the values for each key are aggregated using the given reduce function.
mapPartitions and mapPartitionsWithIndex performs a map operation on an entire partition and returns a new RDD.
As per Apache Spark documentation, groupBy returns an RDD of grouped items where each group consists of a key and a sequence of elements.
groupByKey([numPartitions]) is called on a dataset of (K, V) pairs, and returns a dataset of (K, Iterable) pairs.
As per Apache Spark, filter(function) returns a new dataset formed by selecting those elements of the source on which function returns true.