Interface Aggregator<V,R,A extends Aggregator<V,R,A>>
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Serializable
public interface Aggregator<V,R,A extends Aggregator<V,R,A>> extends Serializable
Behavior of a user-defined Derby aggregator. Aggregates values of type V and returns a result of type R. In addition to the methods in the interface, implementing classes must have a 0-arg public constructor.
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Method Summary
All Methods Instance Methods Abstract Methods Modifier and Type Method Description void
accumulate(V value)
Accumulate the next scalar valuevoid
init()
Initialize the Aggregatorvoid
merge(A otherAggregator)
For merging another partial result into this Aggregator.R
terminate()
Return the result scalar value
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Method Detail
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init
void init()
Initialize the Aggregator
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accumulate
void accumulate(V value)
Accumulate the next scalar value- Parameters:
value
- The new value to accumulate.
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merge
void merge(A otherAggregator)
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
- Parameters:
otherAggregator
- The other Aggregator to merge with
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terminate
R terminate()
Return the result scalar value- Returns:
- the result scalar value
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