org.apache.derby.impl.sql.execute
Class AvgAggregator

java.lang.Object
  org.apache.derby.impl.sql.execute.SystemAggregator
      org.apache.derby.impl.sql.execute.OrderableAggregator
          org.apache.derby.impl.sql.execute.SumAggregator
              org.apache.derby.impl.sql.execute.AvgAggregator

All Implemented Interfaces:

java.io.Externalizable, java.io.Serializable, Formatable, TypedFormat, ExecAggregator

public final class AvgAggregator
extends SumAggregator

Aggregator for AVG(). Extends the SumAggregator and implements a count. Result is then sum()/count(). To handle overflow we catch the exception for value out of range, then we swap the holder for the current sum to one that can handle a larger range. Eventually a sum may end up in a SQLDecimal which can handle an infinite range. Once this type promotion has happened, it will not revert back to the original type, even if the sum would fit in a lesser type.

See Also:

Serialized Form

Field Summary

private long	count
private int	scale

Fields inherited from class org.apache.derby.impl.sql.execute.OrderableAggregator

value

Constructor Summary

AvgAggregator()

Method Summary

protected void	accumulate(DataValueDescriptor addend) Accumulate
DataValueDescriptor	getResult() Return the result of the aggregation.
int	getTypeFormatId() Get the formatID which corresponds to this class.
void	merge(ExecAggregator addend) Merges one aggregator into a another aggregator.
ExecAggregator	newAggregator() Return a new initialized copy of this aggregator, any state set by the setup() method of the original Aggregator must be copied into the new aggregator.
void	readExternal(java.io.ObjectInput in)
void	writeExternal(java.io.ObjectOutput out) Although we are not expected to be persistent per se, we may be written out by the sorter temporarily.

Methods inherited from class org.apache.derby.impl.sql.execute.SumAggregator

toString

Methods inherited from class org.apache.derby.impl.sql.execute.OrderableAggregator

setup

Methods inherited from class org.apache.derby.impl.sql.execute.SystemAggregator

accumulate, didEliminateNulls

Methods inherited from class java.lang.Object

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

Field Detail

count

private long count

scale

private int scale

Constructor Detail

AvgAggregator

public AvgAggregator()

Method Detail

accumulate

protected void accumulate(DataValueDescriptor addend)
                   throws StandardException

Description copied from class: SumAggregator

Accumulate

Overrides:

accumulate in class SumAggregator

Parameters:

addend - value to be added in

Throws:

StandardException - on error

See Also:

ExecAggregator.accumulate(org.apache.derby.iapi.types.DataValueDescriptor, java.lang.Object)

merge

public void merge(ExecAggregator addend)
           throws StandardException

Description copied from interface: ExecAggregator

Merges one aggregator into a another aggregator. Merges two partial aggregates results into a single result. Needed for:

• parallel aggregation
• vector aggregation (GROUP BY)
• distinct aggregates (e.g. MAX(DISTINCT Col))

An example of a merge would be: given two COUNT() aggregators, C1 and C2, a merge of C1 into C2 would set C1.count += C2.count. So, given a CountAggregator with a getCount() method that returns its counts, its merge method might look like this:

                public void merge(ExecAggregator inputAggregator) throws StandardException
                {
                   count += ((CountAccgregator)inputAggregator).getCount();
                } 

Specified by:

merge in interface ExecAggregator

Overrides:

merge in class OrderableAggregator

Parameters:

addend - the other Aggregator (input partial aggregate)

Throws:

StandardException - on error

See Also:

ExecAggregator.merge(org.apache.derby.iapi.sql.execute.ExecAggregator)

getResult

public DataValueDescriptor getResult()
                              throws StandardException

Return the result of the aggregation. If the count is zero, then we haven't averaged anything yet, so we return null. Otherwise, return the running average as a double.

Specified by:

getResult in interface ExecAggregator

Overrides:

getResult in class OrderableAggregator

Returns:

null or the average as Double

Throws:

StandardException - on error

newAggregator

public ExecAggregator newAggregator()

Description copied from interface: ExecAggregator

Return a new initialized copy of this aggregator, any state set by the setup() method of the original Aggregator must be copied into the new aggregator.

Specified by:

newAggregator in interface ExecAggregator

Overrides:

newAggregator in class SumAggregator

Returns:

ExecAggregator the new aggregator

writeExternal

public void writeExternal(java.io.ObjectOutput out)
                   throws java.io.IOException

Description copied from class: OrderableAggregator

Although we are not expected to be persistent per se, we may be written out by the sorter temporarily. So we need to be able to write ourselves out and read ourselves back in. We rely on formatable to handle situations where value is null.

Why would we be called to write ourselves out if we are null? For scalar aggregates, we don't bother setting up the aggregator since we only need a single row. So for a scalar aggregate that needs to go to disk, the aggregator might be null.

Specified by:

writeExternal in interface java.io.Externalizable

Overrides:

writeExternal in class OrderableAggregator

Throws:

java.io.IOException - on error

See Also:

Externalizable.writeExternal(java.io.ObjectOutput)

readExternal

public void readExternal(java.io.ObjectInput in)
                  throws java.io.IOException,
                         java.lang.ClassNotFoundException

Specified by:

readExternal in interface java.io.Externalizable

Overrides:

readExternal in class OrderableAggregator

Throws:

java.io.IOException - on error

java.lang.ClassNotFoundException - on error

See Also:

Externalizable.readExternal(java.io.ObjectInput)

getTypeFormatId

public int getTypeFormatId()

Get the formatID which corresponds to this class.

Specified by:

getTypeFormatId in interface TypedFormat

Overrides:

getTypeFormatId in class SumAggregator

Returns:

the formatID of this class