Note:  Function definitions for count(), max(), min() and sum() are already included in Ingres, in the same sense that arithmetic and comparison operations are pre-defined for user-defined data types; however, avg() is NOT pre-defined. Users can also code their own aggregate functions with distinct function names. The IIADD_FO_DFN structure instance must define “fod_type” to be II_AGGREGATE in this case.

Note:  Like all functions on user-defined data types, aggregate functions must also include definitions of each function instance for the function on a specific user-defined type. These are defined by IIADD_FI_DFN structure instances, and for aggregate functions they must include an “fid_optype” of II_AGGREGATE.

Note:  The instances of the function instance structure must be in a particular sorted sequence. Function instances with fid_optype of II_AGGREGATE must be placed between those for II_OPERATOR and II_NORMAL. If the function instance is for one of the standard Ingres aggregates, the “fid_opid” field must be set to the appropriate code (II_COUNT_OP for COUNT, II_MAX_OP for MAX, II_MIN_OP for MIN, II_SUM_OP for SUM). If the function instance is for a user-defined aggregate function, the fid_opid field must contain the value from the “fod_id” field of the corresponding IIADD_FO_DFN structure instance.

Note:  The function instance definitions must not use the II_RES_EXTERN value for the “fid_rltype” field.