AggregateDefinition

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org.apache.derby.impl.sql.compile
Interface AggregateDefinition

All Known Implementing Classes:: CountAggregateDefinition, MaxMinAggregateDefinition, SumAvgAggregateDefinition

interface AggregateDefinition

An AggregateDefinition defines an aggregate. It is used by Cloudscape during query compilation to determine what Aggregator is used to aggregate a particular data type and what datatype the Aggregator will emit. A single AggregateDefinition may map to one or more Aggregators depending on the input type. For example, a user defined STDEV aggregate may use one aggregator implementation for the INTEGER type and another for a user defined type that implements a point. In this case, both the aggregators would have a single AggregateDefinition that would chose the appropriate aggregator based on the input type. On the other hand, if only a single aggregator is needed to aggregate over all of the input types (e.g. COUNT()), then it may be convenient to implement both the AggregateDefinition and the Aggregator interfaces by the same class.

IBM Corp. reserves the right to change, rename, or remove this interface at any time.

Method Summary

org.apache.derby.catalog.TypeDescriptor getAggregator(org.apache.derby.catalog.TypeDescriptor inputType, java.lang.StringBuffer aggregatorClassName)
Get the aggregator that performs the aggregation on the input datatype at execution time.

Method Detail

getAggregator

public org.apache.derby.catalog.TypeDescriptor getAggregator(org.apache.derby.catalog.TypeDescriptor inputType,
                                                             java.lang.StringBuffer aggregatorClassName)
                                                      throws java.sql.SQLException

Get the aggregator that performs the aggregation on the input datatype at execution time. If the input type can be handled, return a type descriptor with the resultant type information and fill in the string buffer with the name of the class that is used to perform the aggregation over the input type. If the aggregate cannot be performed on this type, then a null should be returned.

The aggregator class must implement a zero argument constructor. The aggregator class can be the same class as the AggregateDefinition if it implements both interfaces.

The result datatype may be the same as the input datatype or a different datatype. To create your own type descriptor to return to this method, see com.ibm.db2j.types.TypeFactory.