Apache PIG中的领先/滞后函数

Question

apache PIG中是否有类似于SQL中的Lead/Lag函数的函数？或者任何可以回溯上一行记录的 pig 函数？

Answer 1

是的，有预定义的功能。请参阅 Piggybank 中的 Over（） 和 Stitch（） 方法。Over（） 在文档中列出了示例。

Answer 2

这是一个替代方案：

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.apache.pig.EvalFunc;
import org.apache.pig.data.DataType;
import org.apache.pig.data.Tuple;
import org.apache.pig.data.TupleFactory;
import org.apache.pig.impl.logicalLayer.FrontendException;
import org.apache.pig.impl.logicalLayer.schema.Schema;
import org.apache.pig.impl.logicalLayer.schema.Schema.FieldSchema;
public class GenericLag2 extends EvalFunc<Tuple>{
    private List<String> lagObjects = null;
    @Override
    public Tuple exec(Tuple input) throws IOException {
        if (lagObjects == null) {
            lagObjects = new ArrayList<String>();
            return null;
        }
        try {
            Tuple output = TupleFactory.getInstance().newTuple(lagObjects.size());
            for (int i = 0; i < lagObjects.size(); i++) {
                output.set(i, lagObjects.get(i));
            }
            lagObjects.clear();
            for (int i = 0; i < input.size(); i++) {
                lagObjects.add(input.get(i).toString());
            }
            return output;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
    @Override
    public Schema outputSchema(Schema input) {
        Schema tupleSchema = new Schema();
        try {
            for (int i = 0; i < input.size(); i++) {
                tupleSchema.add(new FieldSchema("lag_" + i, DataType.CHARARRAY));
            }
            return new Schema(new FieldSchema(getSchemaName(this.getClass().getName().toLowerCase(), input), tupleSchema, DataType.TUPLE));
        } catch (FrontendException e) {
            e.printStackTrace();
            return null;
        }
    }
}

我认为这会更快，但我不确定，因为您必须执行以下操作：

... 
C = ORDER A BY important_order_by_field, second_important_order_by_field
D = FOREACH B GENERATE
    important_order_by_field
    ,second_important_order_by_field
    ,...
    ,FLATTEN(LAG(
        string_field_to_lag
        ,int_field_to_lag
        ,date_field_to_lag
    ))
    ;
E = FOREACH D GENERATE
    important_order_by_field
    ,second_important_order_by_field
    ,...
    ,string_field_to_lag
    ,(int) int_field_to_lag
    ,(date_field_to_lag IS NULL ? 
        null :
        ToDate(SUBSTRING(REPLACE(date_field_to_lag, 'T', ' '), 0, 19),  'yyyy-MM-dd HH:mm:ss')) 
        as date_field_to_lag
;
DUMP E;

Answer 3

好的，

这是我第一次尝试。请注意，我今天刚刚开始学习如何编写UDF。

Maven的pom.xml文件包含：

<dependency>
  <groupId>org.apache.hadoop</groupId>
  <artifactId>hadoop-client</artifactId>
  <version>2.0.0-cdh4.1.0</version>
</dependency>
...

Java UDF 类：

import java.io.IOException;
import org.apache.pig.EvalFunc;
import org.apache.pig.data.Tuple;
public class GenericLag extends EvalFunc<String>{
    private String lagObject = null;
    @Override
    public String exec(Tuple input) throws IOException {
        try {
            String returnObject = getLagObject();
            setLagObject(input.get(0).toString());
            return returnObject;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
    public String getLagObject() {
        return lagObject;
    }
    public void setLagObject(String lagObject) {
        this.lagObject = lagObject;
    }
}

最初，我使用了Object而不是String您在上面看到"字符串"的任何地方，但我收到了此错误：

ERROR org.apache.pig.tools.grunt.Grunt - ERROR 2080: Foreach currently does not handle type Unknown

我必须发出setLagObject(input.get(0).toString());而不是setLagObject(input.get(0);否则我会收到以下错误：

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.String

java.lang.ClassCastException: org.joda.time.DateTime cannot be cast to java.lang.String

以下是我在 Pig 中使用它的方式：

REGISTER /path/to/compiled/file.jar
DEFINE LAG fully.qualified.domain.name.GenericLag();
A = LOAD '/hdfs/path/to/directory' USING PigStorage(',') AS (
    important_order_by_field:int
    ,second_important_order_by_field:string
    ,...
    ,string_field_to_lag:string
    ,int_field_to_lag:int
    ,date_field_to_lag:string
);
B = FOREACH A GENERATE
    important_order_by_field
    ,second_important_order_by_field
    ,...
    ,string_field_to_lag
    ,int_field_to_lag
    ,ToDate(date_field_to_lag, 'yyyy-MM-dd HH:mm:ss')
    ;
C = ORDER A BY important_order_by_field, second_important_order_by_field
D = FOREACH B GENERATE
    important_order_by_field
    ,second_important_order_by_field
    ,...
    ,LAG(string_field_to_lag) AS lag_string
    ,(int) LAG(int_field_to_lag) AS lag_int
    ,(date_field_to_lag IS NULL ? 
        null : 
        ToDate(SUBSTRING(REPLACE(LAG(date_field_to_lag), 'T', ' ') ,0,19), 'yyyy-MM-dd HH:mm:ss')) AS lag_date
    ;
DUMP D;

如果我像这样做最后一行：

ToDate(SUBSTRING(REPLACE(LAG(date_field_to_lag), 'T', ' ') ,0,19), 'yyyy-MM-dd HH:mm:ss') AS lag_date

它将返回以下错误

ERROR org.apache.pig.tools.grunt.Grunt - ERROR 1066: Unable to open iterator for alias LAGGED_RHODES. Backend error : null

检查日志时显示：

java.lang.NullPointerException at org.joda.time.format.DateTimeFormatterBuilder$NumberFormatter.parseInto(DateTimeFormatterBuilder.java:1200)

因为第一行将包含空值。