Hive自定义聚合函数UDAF(计算中位数)

背景需求

中位数（Median）又称中值，统计学中的专有名词，是按顺序排列的一组数据中居于中间位置的数，代表一个样本、种群或概率分布中的一个数值，其可将数值集合划分为相等的上下两部分。对于有限的数集，可以通过把所有观察值高低排序后找出正中间的一个作为中位数。如果观察值有偶数个，通常取最中间的两个数值的平均数作为中位数。

准备1～7个乱序数字

奇数个数字经过排序【1，2，3，4，5，6，7】取出中间一个数字是4，则中位数是4

再增加一个数字8，依然是乱序

偶数个数字经过排序【1，2，3，4，5，6，7，8】取出中间两个数字【4，5】之和除以2则中位数是4.5

 

代码实现

package udf;

import com.google.common.collect.Lists;
import org.apache.hadoop.hive.ql.exec.UDFArgumentTypeException;
import org.apache.hadoop.hive.ql.metadata.HiveException;
import org.apache.hadoop.hive.ql.parse.SemanticException;
import org.apache.hadoop.hive.ql.udf.generic.AbstractGenericUDAFResolver;
import org.apache.hadoop.hive.ql.udf.generic.GenericUDAFEvaluator;
import org.apache.hadoop.hive.serde2.objectinspector.*;
import org.apache.hadoop.hive.serde2.typeinfo.PrimitiveTypeInfo;
import org.apache.hadoop.hive.serde2.typeinfo.TypeInfo;
import org.apache.hadoop.io.DoubleWritable;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

/**
 * @ClassName: UDAFNumericMedian
 * @Description: 
 * @Author: xuezhouyi
 * @Version: V1.0
 **/
public class UDAFNumericMedian extends AbstractGenericUDAFResolver {

	/* 该方法会根据sql传入的参数数据格式指定调用哪个Evaluator进行处理 */
	@Override
	public GenericUDAFEvaluator getEvaluator(TypeInfo[] parameters) throws SemanticException {
		if (parameters.length != 1) {
			throw new UDFArgumentTypeException(parameters.length - 1, "Exactly one argument is expected.");
		}
		if (parameters[0].getCategory() != ObjectInspector.Category.PRIMITIVE) {
			throw new UDFArgumentTypeException(0, "Only primitive type arguments are accepted.");
		}
		if (((PrimitiveTypeInfo) parameters[0]).getPrimitiveCategory() != PrimitiveObjectInspector.PrimitiveCategory.DOUBLE) {
			throw new UDFArgumentTypeException(0, "Only Double type arguments are accepted.");
		}
		return new DoubleEvaluator();
	}

	private static class DoubleEvaluator extends GenericUDAFEvaluator {
		private PrimitiveObjectInspector inputOI;
		private StandardListObjectInspector mergeOI;

		/* 确定各个阶段输入输出参数的数据格式ObjectInspectors */
		@Override
		public ObjectInspector init(GenericUDAFEvaluator.Mode m, ObjectInspector[] parameters) throws HiveException {
			super.init(m, parameters);
			if (m == Mode.PARTIAL1 || m == Mode.COMPLETE) {
				inputOI = (PrimitiveObjectInspector) parameters[0];
				return ObjectInspectorFactory.getStandardListObjectInspector(ObjectInspectorUtils.getStandardObjectInspector(inputOI));
			} else {
				if (parameters[0] instanceof StandardListObjectInspector) {
					mergeOI = (StandardListObjectInspector) parameters[0];
					inputOI = (PrimitiveObjectInspector) mergeOI.getListElementObjectInspector();
					return ObjectInspectorUtils.getStandardObjectInspector(mergeOI);
				} else {
					inputOI = (PrimitiveObjectInspector) parameters[0];
					return ObjectInspectorFactory.getStandardListObjectInspector(ObjectInspectorUtils.getStandardObjectInspector(inputOI));
				}
			}
		}

		/* 保存数据聚集结果的类 */
		private static class MyAggBuf implements AggregationBuffer {
			List