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

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

All Implemented Interfaces:

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

Direct Known Subclasses:

MaxMinAggregator, SumAggregator

abstract class OrderableAggregator
extends SystemAggregator

Abstract aggregator for Orderable aggregates (max/min).

Field Summary

protected DataValueDescriptor

value

Constructor Summary

OrderableAggregator()

Method Summary

DataValueDescriptor	getResult() Return the result of the operations that we have been performing.
void	merge(ExecAggregator addend) Merges one aggregator into a another aggregator.
void	readExternal(java.io.ObjectInput in)
void	setup(ClassFactory cf, java.lang.String aggregateName, DataTypeDescriptor returnDataType) Set's up the aggregate for processing.
java.lang.String	toString()
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.SystemAggregator

accumulate, accumulate, didEliminateNulls

Methods inherited from class java.lang.Object

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

Methods inherited from interface org.apache.derby.iapi.sql.execute.ExecAggregator

newAggregator

Methods inherited from interface org.apache.derby.iapi.services.io.TypedFormat

getTypeFormatId

Field Detail

value

protected DataValueDescriptor value

Constructor Detail

OrderableAggregator

OrderableAggregator()

Method Detail

setup

public void setup(ClassFactory cf,
                  java.lang.String aggregateName,
                  DataTypeDescriptor returnDataType)

Description copied from interface: ExecAggregator

Set's up the aggregate for processing.

Parameters:

cf - Database-specific class factory.

aggregateName - For builtin aggregates, this is a SQL aggregate name like MAX. For user-defined aggregates, this is the name of the user-written class which implements org.apache.derby.agg.Aggregator.

returnDataType - The type returned by the getResult() method.

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();
                } 

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 operations that we have been performing. Returns a DataValueDescriptor.

Returns:

the result as a DataValueDescriptor

Throws:

StandardException - on error

toString

public java.lang.String toString()

Overrides:

toString in class SystemAggregator

writeExternal

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

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 SystemAggregator

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 SystemAggregator

Throws:

java.io.IOException - on error

java.lang.ClassNotFoundException - on error

See Also:

Externalizable.readExternal(java.io.ObjectInput)