Hive_ function (D)

1: Built-in functions

hive there are many built-in functions, as a set of functions comes mysql: The nvl (), sum (), avg (), etc.

 

2: Hive UDF custom function development

UDF acting on the single data line, a data line is generated as output. (Mathematical functions, string functions)

UDAF (user-defined aggregate function): receiving a plurality of input data lines, and generating an output data line. (Count, max)

Development Example

1, first developed a java class that inherits UDF, and override evaluate methods

package cn.itcast.bigdata.udf
import org.apache.hadoop.hive.ql.exec.UDF;
import org.apache.hadoop.io.Text;

public final class Lower extends UDF{
	public Text evaluate(final Text s){
		if(s==null){return null;}
		return new Text(s.toString().toLowerCase());
	}
}

2, labeled jar package uploaded to the server

3, the packet is added to the jar the hive classpath

hive>add JAR /home/hadoop/udf.jar;

4. Create a temporary function associated with good java class and Development

Hive>create temporary function tolowercase as 'cn.itcast.bigdata.udf.ToProvince';

3: Hive Transform achieve

Hive of TRANSFORM keyword provides call from a script written in SQL functions, suitable for the realization Hive features not not write UDF situation.

Example:

CREATE TABLE u_data_new (
  movieid INT,
  rating INT,
  weekday INT,
  userid INT)
ROW FORMAT DELIMITED
FIELDS TERMINATED BY '\t';

add FILE weekday_mapper.py;

INSERT OVERWRITE TABLE u_data_new
SELECT
  TRANSFORM (movieid , rate, timestring,uid)
  USING 'python weekday_mapper.py'
  AS (movieid, rating, weekday,userid)
FROM t_rating;

Which reads as follows weekday_mapper.py

#!/bin/python
import sys
import datetime

for line in sys.stdin:
  line = line.strip()
  movieid, rating, unixtime,userid = line.split('\t')
  weekday = datetime.datetime.fromtimestamp(float(unixtime)).isoweekday()
  print '\t'.join([movieid, rating, str(weekday),userid])