Reynold Xin created SPARK-15598:
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Summary: Change Aggregator.zero to Aggregator.init
Key: SPARK-15598
URL: https://issues.apache.org/jira/browse/SPARK-15598
Project: Spark
Issue Type: Sub-task
Components: SQL
Reporter: Reynold Xin
org.apache.spark.sql.expressions.Aggregator currently requires defining the
zero value for an aggregator. This is actually a limitation making it difficult
to implement APIs such as reduce. In reduce (or reduceByKey), a single
associative and commutative reduce function is specified by the user, and there
is no definition of zero value.
A small tweak to the API is to change zero to init, taking an input, similar to
the following:
{code}
abstract class Aggregator[-IN, BUF, OUT] extends Serializable {
def init(a: IN): BUF
def reduce(b: BUF, a: IN): BUF
def merge(b1: BUF, b2: BUF): BUF
def finish(reduction: BUF): OUT
}
{code}
Then reduce can be implemented using:
{code}
f: (T, T) => T
new Aggregator[T, T, T] {
override def init(a: T): T = identify
override def reduce(b: T, a: T): T = f(b, a)
override def merge(b1: T, b2: T): T = f(b1, b2)
override def finish(reduction: T): T = identify
}
{code}
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