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 mozIStorageAggregateFunction represents aggregate SQL function.
 Common examples of aggregate functions are SUM() and COUNT().

 An aggregate function calculates one result for a given set of data, where
 a set of data is a group of tuples. There can be one group
 per request or many of them, if GROUP BY clause is used or not.
[scriptable, uuid(763217b7-3123-11da-918d-000347412e16)]
interface mozIStorageAggregateFunction : nsISupports

Methods

 Called when all tuples in a group have been processed and the engine
 needs the aggregate function's value.

 @returns aggregate result as Variant.
nsIVariant onFinal() 
 onStep is called when next value should be passed to
 a custom function.
 
 @param aFunctionArguments    The arguments passed in to the function
void onStep(in mozIStorageValueArray aFunctionArguments)