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https://issues.apache.org/jira/browse/DERBY-672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13416960#comment-13416960
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Knut Anders Hatlen commented on DERBY-672:
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The proposal looks good to me. I would be fine with either of the two
alternatives, but I agree that alternative (ii) sounds more elegant. It might
also be more flexible, as adding optional clauses to the syntax later is
probably easier than adding more parameters to existing procedures.
I have no objections to restricting new features to modern platforms. By using
parameterized types, the user doesn't have to cast the parameter in the
accumulate() method, and it is checked at compile time that the value returned
from terminate() has the correct type.
It does however still seem to me that the user will have to cast the
otherAggregator parameter that's passed to the merge() method, as the
implementation in most cases will need to access fields/method of the instance
that's passed in. Example (reimplementing the SUM aggregate function for
integers):
public class MyIntSum implements Aggregator<Integer,Integer> {
private int sum;
public void init() { sum=0; }
public void accumulate(Integer value) { sum += value; }
public void merge(Aggregator<Integer,Integer> otherAggregator) {
MyIntSum that = (MyIntSum) otherAggregator;
this.sum += that.sum;
}
public Integer terminate() { return sum; }
}
Not sure what's the cleanest way to handle this. Maybe we could add a third
parameterized type that tells which kind of aggregators it can be merged with.
That would mean changing the interface declaration like this:
public interface Aggregator<V, R, T extends Aggregator<V,R,T>> extends
Serializable
{
...
public void merge( T otherAggregator ) throws SQLException;
...
}
and MyIntSum would become:
public class MyIntSum implements Aggregator<Integer,Integer,MyIntSum> {
private int sum;
public void init() { sum=0; }
public void accumulate(Integer value) { sum += value; }
public void merge(MyIntSum that) { this.sum += that.sum; }
public Integer terminate() { return sum; }
}
It makes the declaration a little more messy, but the implementation becomes
more concise and statically type-checked.
> Re-enable user defined aggregates
> ---------------------------------
>
> Key: DERBY-672
> URL: https://issues.apache.org/jira/browse/DERBY-672
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Reporter: Rick Hillegas
>
> Nicolas Dufour in an email thread titled "functions and list" started on
> November 2, 2005 requests the ability to create user defined aggregates.
> This functionality used to be in Cloudscape. It was disabled presumably
> because it was considered non-standard. However, most of the machinery needed
> for this feature is still in the code. We should re-enable user defined
> aggregates after we agree on acceptable syntax.
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