org.apache.pig.piggybank.evaluation
Class Over

java.lang.Object
  org.apache.pig.EvalFunc<DataBag>
      org.apache.pig.piggybank.evaluation.Over

public class Over
extends EvalFunc<DataBag>

Given an aggregate function, a bag, and possibly a window definition, produce output that matches SQL OVER. It is the reponsibility of the caller to have already ordered the bag as required by their operation. The aggregate, and window definition are passed in the constructor. The bag is passed to exec each time.

Usage: Over(bag, function_to_call[, window_start, window_end[, function specific args]])

bag - The bag to be called. Most functions assume this is a bag with tuples of a single field.

function_to_call - Can be one of the following:

• count
• sum(double)
• sum(float)
• sum(int)
• sum(long)
• sum(bytearray)
• avg(double)
• avg(float)
• avg(long)
• avg(int)
• avg(bytearray)
• min(double)
• min(float)
• min(long)
• min(int)
• min(chararray)
• min(bytearray)
• max(double)
• max(float)
• max(long)
• max(int)
• max(chararray)
• max(bytearray)
• row_number
• first_value
• last_value
• lead
• lag
• rank
• dense_rank
• ntile
• percent_rank
• cume_dist

window_start - optional - Record to start window on for the function. -1 indicates 'unbounded preceding', i.e. the beginning of the bag. A positive integer indicates that number of records before the current record. 0 indicates the current record. If not specified -1 is the default.

window_end - optional - Record to end window on for the function. -1 indicates 'unbounded following', i.e. the end of the bag. A positive integer indicates that number of records after the current record. 0 indicates teh current record. If not specified 0 is the default.

function_specific_args - maybe optional - The following functions accept require additional arguments:

• lead - two optional arguments, first number of records ahead of current to lead, second default value when lead extends beyond the end of the window frame.
• lag - two optional arguments, first number of records behind of current to lag, second default value when lag extends beyond the beginning of the window frame.
• rank - one required, the number of the field the bag is ordered by
• dense_rank - one required, the number of the field the bag is ordered by
• ntile - one required, the number of buckets to split the data into
• percent_rank - one required, the number of the field the bag is ordered by
• cume_dist - one required, the number of the field the bag is ordered by

Example Usage:

To do a cumulative sum:

 A = load 'T' AS (si:chararray, i:int, d:long, f:float, s:chararray);
 C = foreach (group A by si) {
     Aord = order A by d;
     generate flatten(Stitch(Aord, Over(Aord.f, 'sum(float)')));
 }
 D = foreach C generate s, $5;

This is equivalent to the SQL statement

select s, sum(f) over (partition by si order by d) from T;

To find the record 3 ahead of the current record, using a window between the current row and 3 records ahead and a default value of 0.

 A = load 'T' AS (si:chararray, i:int, d:long, f:float, s:chararray);
 C = foreach (group A by si) {
     Aord = order A by i;
     generate flatten(Stitch(Aord, Over(Aord.i, 'lead', 0, 3, 3, 0)));
 }
 D = foreach C generate s, $9;

This is equivalent to the SQL statement

select s, lead(i, 3, 0) over (partition by si order by i rows between current row and 3 following) over T;

Over accepts a constructor argument specifying the name and type, colon-separated, of its return schema.

 DEFINE IOver org.apache.pig.piggybank.evaluation.Over('state_rk:int');
 cities = LOAD 'cities' AS (city:chararray, state:chararray, pop:int);
 -- Decorate each city with its population rank within the state it belongs to:
 ranked = FOREACH(GROUP cities BY state) {
   c_ord = ORDER cities BY pop DESC;
   GENERATE FLATTEN(Stitch(c_ord,
     IOver(c_ord, 'rank', -1, -1, 2))); -- beginning (-1) to end (-1) on third field (2)
 };
 DESCRIBE ranked;
 -- ranked: {stitched::city: chararray,stitched::state: chararray,stitched::pop: int,stitched::state_rk: int}
 DUMP ranked;
 -- ...
 -- (Nashville,Tennessee,609644,2)
 -- (Houston,Texas,2145146,1)
 -- (San Antonio,Texas,1359758,2)
 -- (Dallas,Texas,1223229,3)
 -- (Austin,Texas,820611,4)
 -- ...
 

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.pig.EvalFunc

EvalFunc.SchemaType

Field Summary

Fields inherited from class org.apache.pig.EvalFunc

log, pigLogger, reporter

Constructor Summary

Over()

Over(String typespec)

Method Summary

DataBag	exec(Tuple input) This callback method must be implemented by all subclasses.
Schema	outputSchema(Schema inputSch) Report the schema of the output of this UDF.

Methods inherited from class org.apache.pig.EvalFunc

finish, getArgToFuncMapping, getCacheFiles, getInputSchema, getLogger, getPigLogger, getReporter, getReturnType, getSchemaName, getSchemaType, isAsynchronous, progress, setInputSchema, setPigLogger, setReporter, setUDFContextSignature, warn

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Over

public Over()

Over

public Over(String typespec)

Method Detail

exec

public DataBag exec(Tuple input)
             throws IOException

Description copied from class: EvalFunc

This callback method must be implemented by all subclasses. This is the method that will be invoked on every Tuple of a given dataset. Since the dataset may be divided up in a variety of ways the programmer should not make assumptions about state that is maintained between invocations of this method.

Specified by:

exec in class EvalFunc<DataBag>

Parameters:

input - the Tuple to be processed.

Returns:

result, of type T.

Throws:

IOException

outputSchema

public Schema outputSchema(Schema inputSch)

Description copied from class: EvalFunc

Report the schema of the output of this UDF. Pig will make use of this in error checking, optimization, and planning. The schema of input data to this UDF is provided.

The default implementation interprets the OutputSchema annotation, if one is present. Otherwise, it returns null (no known output schema).

Overrides:

outputSchema in class EvalFunc<DataBag>

Parameters:

inputSch - Schema of the input

Returns:

Schema of the output