ExecAggregator

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org.apache.derby.iapi.sql.execute
Interface ExecAggregator

All Superinterfaces:: java.io.Externalizable, org.apache.derby.iapi.services.io.Formatable, java.io.Serializable, org.apache.derby.iapi.services.io.TypedFormat

public interface ExecAggregator
extends org.apache.derby.iapi.services.io.Formatable

An ExecAggregator is the interface that execution uses to an aggregate. System defined aggregates will implement this directly.

The life time of an ExecAggregator is as follows.

An ExecAggregator instance is created using the defined class name.
Its setup() method is called to define its role (COUNT(*), SUM, etc.).
Its newAggregator() method may be called any number of times to create new working aggregators as required. These aggregators have the same role and must be created in an initialized state.
accumlate and merge will be called across these set of aggregators
One of these aggregators will be used as the final one for obtaining the result

Method Summary

void accumulate(org.apache.derby.iapi.types.DataValueDescriptor addend, java.lang.Object ga)
          Iteratively accumulates the addend into the aggregator.

boolean didEliminateNulls()
          Return true if the aggregation eliminated at least one null from the input data set.

org.apache.derby.iapi.types.DataValueDescriptor getResult()
          Produces the result to be returned by the query.

void merge(ExecAggregator inputAggregator)
          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 setup(java.lang.String aggregateName)
          Set's up the aggregate for processing.

Methods inherited from interface java.io.Externalizable

readExternal, writeExternal

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

getTypeFormatId

Method Detail

setup

public void setup(java.lang.String aggregateName)

Set's up the aggregate for processing.

accumulate

public void accumulate(org.apache.derby.iapi.types.DataValueDescriptor addend,
                       java.lang.Object ga)
                throws org.apache.derby.iapi.error.StandardException

Iteratively accumulates the addend into the aggregator. Called on each member of the set of values that is being aggregated.

merge

public void merge(ExecAggregator inputAggregator)
           throws org.apache.derby.iapi.error.StandardException

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();
                }

getResult

public org.apache.derby.iapi.types.DataValueDescriptor getResult()
                                                          throws org.apache.derby.iapi.error.StandardException

Produces the result to be returned by the query. The last processing of the aggregate.

newAggregator

public 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.

didEliminateNulls

public boolean didEliminateNulls()

Return true if the aggregation eliminated at least one null from the input data set.