Class VarPAggregator<V extends Number>

java.lang.Object

org.apache.derby.impl.sql.execute.VarPAggregator<V>

All Implemented Interfaces:

Externalizable, Serializable, Aggregator<V, Double, VarPAggregator<V>>

Direct Known Subclasses:

StdDevPAggregator, VarSAggregator

public class VarPAggregator<V extends Number>
extends Object
implements Aggregator<V, Double, VarPAggregator<V>>, Externalizable

This class implements the SQL Standard VAR_POP() aggregator, computing a population's variance. It uses the IBM formula described here:

sum(xi2)/n - m2

where

n is the number of items in the population
m is the population average
x1 ... xn are the items in the population

The IBM formula can be computed without buffering up an arbitrarily long list of items. The IBM formula is algebraically equivalent to the textbook formula for population variance:

sum( (xi - m)2 )/n

See Also:

• Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static class	VarPAggregator.Sums

Field Summary

Fields

Modifier and Type	Field	Description
protected int	count
private static final long	serialVersionUID
protected VarPAggregator.Sums	sums

Constructor Summary

Constructors

Constructor	Description
VarPAggregator()

Method Summary

Modifier and Type	Method	Description
void	accumulate(V value)	Accumulate the next scalar value
protected Double	computeVar()
void	init()	Initialize the Aggregator
void	merge(VarPAggregator<V> otherAggregator)	For merging another partial result into this Aggregator.
void	readExternal(ObjectInput in)
Double	terminate()	Return the result scalar value
void	writeExternal(ObjectOutput out)

Methods inherited from class Object

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

Field Details

serialVersionUID

private static final long serialVersionUID

See Also:

• Constant Field Values

sums

protected VarPAggregator.Sums sums

count

protected int count

Constructor Details

VarPAggregator

public VarPAggregator()

Method Details

init

public void init()

Description copied from interface: Aggregator

Initialize the Aggregator

Specified by:

init in interface Aggregator<V extends Number, Double, VarPAggregator<V extends Number>>

accumulate

public void accumulate(V value)

Description copied from interface: Aggregator

Accumulate the next scalar value

Specified by:

accumulate in interface Aggregator<V extends Number, Double, VarPAggregator<V extends Number>>

merge

public void merge(VarPAggregator<V> otherAggregator)

Description copied from interface: Aggregator

For merging another partial result into this Aggregator. This lets the SQL interpreter divide the incoming rows into subsets, aggregating each subset in isolation, and then merging the partial results together. This method can be called when performing a grouped aggregation with a large number of groups. While processing such a query, Derby may write intermediate grouped results to disk. The intermediate results may be retrieved and merged with later results if Derby encounters later rows which belong to groups whose intermediate results have been written to disk. This situation can occur with a query like the following:

select a, mode( b ) from mode_inputs group by a order by a

Specified by:

merge in interface Aggregator<V extends Number, Double, VarPAggregator<V extends Number>>

computeVar

protected Double computeVar()

terminate

public Double terminate()

Description copied from interface: Aggregator

Return the result scalar value

Specified by:

terminate in interface Aggregator<V extends Number, Double, VarPAggregator<V extends Number>>

readExternal

public void readExternal(ObjectInput in)
                  throws IOException,
ClassNotFoundException

Specified by:

readExternal in interface Externalizable

Throws:

IOException

ClassNotFoundException

writeExternal

public void writeExternal(ObjectOutput out)
                   throws IOException

Specified by:

writeExternal in interface Externalizable

Throws:

IOException