Class OrderableAggregator

    • 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)
      • 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)