Create aggregates

An aggregate is a function that returns information about a set of query results. For example, the SUM aggregate adds all the query results together and returns the result. An aggregate is invoked in SQL as a single function but is implemented as one or more support functions. You can use BladeSmith to create user-defined aggregates that implement user-defined routines.

Restriction: You cannot use the Aggregate wizard to overload built-in aggregates for extended data types; you must use the Routine wizard to overload each of the operators required by the built-in aggregate. For more information about the Routine wizard, see Create routines. For more information about built-in aggregates, see the HCL OneDB™ DataBlade API Programmer's Guide.

You can define two aggregates that have the same name but operate on different data types. An aggregate acts as a template: the aggregate support functions must have the same names for both aggregates. If you overload an aggregate, you cannot add, remove, or change the names of its support functions. Use your new object prefix to begin the name of your aggregate to avoid accidentally overloading an aggregate in another DataBlade module.

The following table describes the properties you specify when you create an aggregate.

Table 1. Properties for creating an aggregate
Property	Default value	Description
Aggregate name	Aggregate	The name of the aggregate function. If you are overloading an aggregate, the name can be the name of an existing aggregate; otherwise, the name must be unique. New aggregate names must begin with the new object prefix. See Aggregate name for more information.
Language	C	Which language to use for the aggregate functions: C or Java™. You must set server compatibility to 9.2 or later to generate code for Java projects. You need J/Foundation to enable Java services.
Iteration type	None	The data type on which the aggregate function operates. See Iteration type for restrictions.
Initialization parameter	No	Optional. Used only for aggregates whose behavior can be changed dynamically. See Initialization parameter for more information.
Return type	None	The data type of the result of the aggregate function.
State type	None	The data type of the intermediate aggregation state. The state type is often POINTER. See State type for more information.
Initialization function	AggregateInit	The function called before the aggregation begins. Not required if the state and iteration data types are the same and there is no initialization parameter. See Initialization function for more information.
Iteration function	AggregateIter	Called once for every value that is aggregated. By default, this function accepts null values. See Iteration function for more information.
Combine function	AggregateComb	Optional. Merges results from parallel iterations. See Combine function for more information.
Final function	AggregateFinl	Performs computations on the combined state, cleans up memory, and returns the final value. See Final function for more information.

For information about how aggregates behave, see the HCL OneDB DataBlade API Programmer's Guide.