[SPARK-18980][SQL] implement Aggregator with TypedImperativeAggregate

## What changes were proposed in this pull request?

Currently we implement `Aggregator` with `DeclarativeAggregate`, which will serialize/deserialize the buffer object every time we process an input.

This PR implements `Aggregator` with `TypedImperativeAggregate` and avoids to serialize/deserialize buffer object many times. The benchmark shows we get about 2 times speed up.

For simple buffer object that doesn't need serialization, we still go with `DeclarativeAggregate`, to avoid performance regression.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16383 from cloud-fan/aggregator.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/TestingTypedCount.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/TestingTypedCount.scala
index d27287bad0..aaf1db65a6 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/TestingTypedCount.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/TestingTypedCount.scala
@@ -46,14 +46,16 @@ case class TestingTypedCount(
 
   override def createAggregationBuffer(): State = TestingTypedCount.State(0L)
 
-  override def update(buffer: State, input: InternalRow): Unit = {
+  override def update(buffer: State, input: InternalRow): State = {
     if (child.eval(input) != null) {
       buffer.count += 1
     }
+    buffer
   }
 
-  override def merge(buffer: State, input: State): Unit = {
+  override def merge(buffer: State, input: State): State = {
     buffer.count += input.count
+    buffer
   }
 
   override def eval(buffer: State): Any = buffer.count