Apache Pig - TOP()


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The TOP() function of Pig Latin is used to get the top N tuples of a bag. To this function, as inputs, we have to pass a relation, the number of tuples we want, and the column name whose values are being compared. This function will return a bag containing the required columns.

Syntax

Given below is the syntax of the function TOP().

grunt> TOP(topN,column,relation)

Example

Assume we have a file named employee_details.txt in the HDFS directory /pig_data/, with the following content.

employee_details.txt

001,Robin,22,newyork 
002,BOB,23,Kolkata 
003,Maya,23,Tokyo 
004,Sara,25,London 
005,David,23,Bhuwaneshwar 
006,Maggy,22,Chennai 
007,Robert,22,newyork 
008,Syam,23,Kolkata 
009,Mary,25,Tokyo 
010,Saran,25,London 
011,Stacy,25,Bhuwaneshwar 
012,Kelly,22,Chennai

We have loaded this file into Pig with the relation name emp_data as shown below.

grunt> emp_data = LOAD 'hdfs://localhost:9000/pig_data/ employee_details.txt' USING PigStorage(',')
   as (id:int, name:chararray, age:int, city:chararray);

Group the relation emp_data by age, and store it in the relation emp_group.

grunt> emp_group = Group emp_data BY age;

Verify the relation emp_group using the Dump operator as shown below.

grunt> Dump emp_group;
  
(22,{(12,Kelly,22,Chennai),(7,Robert,22,newyork),(6,Maggy,22,Chennai),(1,Robin, 22,newyork)}) 
(23,{(8,Syam,23,Kolkata),(5,David,23,Bhuwaneshwar),(3,Maya,23,Tokyo),(2,BOB,23, Kolkata)}) 
(25,{(11,Stacy,25,Bhuwaneshwar),(10,Saran,25,London),(9,Mary,25,Tokyo),(4,Sara, 25,London)})

Now, you can get the top two records of each group arranged in ascending order (based on id) as shown below.

grunt> data_top = FOREACH emp_group { 
   top = TOP(2, 0, emp_data); 
   GENERATE top; 
}

In this example we are retriving the top 2 tuples of a group having greater id. Since we are retriving top 2 tuples basing on the id, we are passing the index of the column name id as second parameter of TOP() function.

Verification

You can verify the contents of the data_top relation using the Dump operator as shown below.

grunt> Dump data_top;
  
({(7,Robert,22,newyork),(12,Kelly,22,Chennai)}) 
({(5,David,23,Bhuwaneshwar),(8,Syam,23,Kolkata)}) 
({(10,Saran,25,London),(11,Stacy,25,Bhuwaneshwar)})
apache_pig_bag_tuple_functions.htm
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