org.apache.pig.builtin
Class COUNT

java.lang.Object
  org.apache.pig.EvalFunc<Long>
      org.apache.pig.builtin.COUNT

All Implemented Interfaces:

Accumulator<Long>, Algebraic

public class COUNT
extends EvalFunc<Long>
implements Algebraic, Accumulator<Long>

Generates the count of the number of values in a bag. This count does not include null values, and thus matches SQL semantics for COUNT(a) (where a is field) but not for COUNT(*) (where * in SQL indicates all).

This class implements Algebraic, so if possible the execution will performed in a distributed fashion.

There are no restrictions as to the data types inside the bag to be counted.

COUNT implements the Accumulator interface as well. While this will never be the preferred method of usage it is available in case the combiner can not be used for a given calculation.

Nested Class Summary

static class	COUNT.Final
static class	COUNT.Initial
static class	COUNT.Intermediate

Field Summary

Fields inherited from class org.apache.pig.EvalFunc

log, pigLogger, reporter, returnType

Constructor Summary

COUNT()

Method Summary

void	accumulate(Tuple b) Pass tuples to the UDF.
void	cleanup() Called after getValue() to prepare processing for next key.
Long	exec(Tuple input) This callback method must be implemented by all subclasses.
List<FuncSpec>	getArgToFuncMapping() Allow a UDF to specify type specific implementations of itself.
String	getFinal() Get the final function.
String	getInitial() Get the initial function.
String	getIntermed() Get the intermediate function.
Long	getValue() Called when all tuples from current key have been passed to accumulate.
Schema	outputSchema(Schema input) Report the schema of the output of this UDF.
protected static Long	sum(Tuple input)

Methods inherited from class org.apache.pig.EvalFunc

finish, getCacheFiles, getLogger, getPigLogger, getReporter, getReturnType, getSchemaName, isAsynchronous, progress, setPigLogger, setReporter, warn

Methods inherited from class java.lang.Object

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

Constructor Detail

COUNT

public COUNT()

Method Detail

exec

public Long 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<Long>

Parameters:

input - the Tuple to be processed.

Returns:

result, of type T.

Throws:

IOException

getInitial

public String getInitial()

Description copied from interface: Algebraic

Get the initial function.

Specified by:

getInitial in interface Algebraic

Returns:

A function name of f_init. f_init should be an eval func.

getIntermed

public String getIntermed()

Description copied from interface: Algebraic

Get the intermediate function.

Specified by:

getIntermed in interface Algebraic

Returns:

A function name of f_intermed. f_intermed should be an eval func.

getFinal

public String getFinal()

Description copied from interface: Algebraic

Get the final function.

Specified by:

getFinal in interface Algebraic

Returns:

A function name of f_final. f_final should be an eval func parametrized by the same datum as the eval func implementing this interface.

sum

protected static Long sum(Tuple input)
                   throws ExecException,
                          NumberFormatException

Throws:

ExecException

NumberFormatException

outputSchema

public Schema outputSchema(Schema input)

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.

Overrides:

outputSchema in class EvalFunc<Long>

Parameters:

input - Schema of the input

Returns:

Schema of the output

getArgToFuncMapping

public List<FuncSpec> getArgToFuncMapping()
                                   throws FrontendException

Description copied from class: EvalFunc

Allow a UDF to specify type specific implementations of itself. For example, an implementation of arithmetic sum might have int and float implementations, since integer arithmetic performs much better than floating point arithmetic. Pig's typechecker will call this method and using the returned list plus the schema of the function's input data, decide which implementation of the UDF to use.

Overrides:

getArgToFuncMapping in class EvalFunc<Long>

Returns:

A List containing FuncSpec objects representing the EvalFunc class which can handle the inputs corresponding to the schema in the objects. Each FuncSpec should be constructed with a schema that describes the input for that implementation. For example, the sum function above would return two elements in its list:

1. FuncSpec(this.getClass().getName(), new Schema(new Schema.FieldSchema(null, DataType.DOUBLE)))
2. FuncSpec(IntSum.getClass().getName(), new Schema(new Schema.FieldSchema(null, DataType.INTEGER)))

This would indicate that the main implementation is used for doubles, and the special implementation IntSum is used for ints.

Throws:

FrontendException

accumulate

public void accumulate(Tuple b)
                throws IOException

Description copied from interface: Accumulator

Pass tuples to the UDF.

Specified by:

accumulate in interface Accumulator<Long>

Parameters:

b - A tuple containing a single field, which is a bag. The bag will contain the set of tuples being passed to the UDF in this iteration.

Throws:

IOException

cleanup

public void cleanup()

Description copied from interface: Accumulator

Called after getValue() to prepare processing for next key.

Specified by:

cleanup in interface Accumulator<Long>

getValue

public Long getValue()

Description copied from interface: Accumulator

Called when all tuples from current key have been passed to accumulate.

Specified by:

getValue in interface Accumulator<Long>

Returns:

the value for the UDF for this key.