hive的 UDAF

Hive的UDAF详解

什么是UDAF？

UDAF（User-Defined Aggregate Function）是Hive中的一种自定义聚合函数，允许用户根据自己的需求定义新的聚合操作。Hive提供了许多内置的聚合函数，如SUM、AVG和COUNT等，但是有时候这些内置函数无法满足我们的需求，这时就需要用到UDAF。

UDAF不同于UDF（User-Defined Function），UDF一般是对单个值进行操作，而UDAF是对一组值进行操作并生成一个结果。UDAF最常见的例子是计算平均值或者求和。

UDAF的使用

使用UDAF的步骤如下：

1. 编写UDAF的Java类，继承Hive提供的UDAF类，并实现相应的聚合操作逻辑。
2. 在Hive中注册UDAF，并使用该函数进行聚合操作。

下面是一个示例，展示如何使用UDAF计算某个数值字段的平均值。

首先，我们需要创建一个Java类AverageUDAF，继承Hive的UDAF类，并实现UDAFEvaluator接口。

import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;
import org.apache.hadoop.hive.ql.udf.generic.AbstractGenericUDAFResolver;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorFactory;
import org.apache.hadoop.hive.serde2.objectinspector.StandardStructObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.primitive.DoubleObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.primitive.PrimitiveObjectInspectorFactory;
import org.apache.hadoop.hive.serde2.objectinspector.primitive.PrimitiveObjectInspectorUtils;

public class AverageUDAF extends AbstractGenericUDAFResolver {

    @Override
    public GenericUDAFEvaluator getEvaluator(GenericUDAFParameterInfo info) {
        return new Evaluator();
    }

    public static class Evaluator implements UDAFEvaluator {

        private DoubleObjectInspector partialResultOI;
        private StandardStructObjectInspector returnOI;
        private DoubleObjectInspector averageFieldOI;

        private double sum;
        private long count;

        @Override
        public ObjectInspector init(Mode m, ObjectInspector[] parameters) throws HiveException {
            // 输入参数的类型检查
            assert (parameters.length == 1);
            assert (parameters[0] instanceof DoubleObjectInspector);

            partialResultOI = (DoubleObjectInspector) parameters[0];

            // 聚合结果的字段
            returnOI = ObjectInspectorFactory.getStandardStructObjectInspector(
                    new ArrayList<>(Arrays.asList("sum", "count")),
                    new ArrayList<>(Arrays.asList(
                            PrimitiveObjectInspectorFactory.writableDoubleObjectInspector,
                            PrimitiveObjectInspectorFactory.writableLongObjectInspector)));

            averageFieldOI = PrimitiveObjectInspectorFactory.writableDoubleObjectInspector;

            sum = 0;
            count = 0;

            return returnOI;
        }

        @Override
        public AggregationBuffer getNewAggregationBuffer() throws HiveException {
            return new AverageAggregationBuffer();
        }

        @Override
        public void reset(AggregationBuffer aggregationBuffer) throws HiveException {
            AverageAggregationBuffer buffer = (AverageAggregationBuffer) aggregationBuffer;
            buffer.sum = 0;
            buffer.count = 0;
        }

        @Override
        public void iterate(AggregationBuffer aggregationBuffer, Object[] objects) throws HiveException {
            AverageAggregationBuffer buffer = (AverageAggregationBuffer) aggregationBuffer;
            double value = PrimitiveObjectInspectorUtils.getDouble(objects[0], partialResultOI);
            buffer.sum += value;
            buffer.count++;
        }

        @Override
        public Object terminatePartial(AggregationBuffer aggregationBuffer) throws HiveException {
            AverageAggregationBuffer buffer = (AverageAggregationBuffer) aggregationBuffer;
            return new Object[]{buffer.sum, buffer.count};
        }

        @Override
        public void merge(AggregationBuffer aggregationBuffer, Object o) throws HiveException {
            if (o == null) {
                return;
            }

            AverageAggregationBuffer buffer = (AverageAggregationBuffer) aggregationBuffer;
            Object[] partial = (Object[]) o;
            buffer.sum += PrimitiveObjectInspectorUtils.getDouble(partial[0], partialResultOI);
            buffer.count += PrimitiveObjectInspectorUtils.getLong(partial[1], partialResultOI);
        }

        @Override
        public Object terminate(AggregationBuffer aggregationBuffer) throws HiveException {
            AverageAggregationBuffer buffer = (AverageAggregationBuffer) aggregationBuffer;
            return buffer.sum / buffer.count;
        }

        public static class AverageAggregationBuffer implements AggregationBuffer {
            double sum;
            long count;
        }
    }
}

接下来，我们需要在Hive中注册这个UDAF。可以在