17.10 - Using a C Structure to Access the Storage Area - Advanced SQL Engine - Teradata Database

Teradata Vantageā„¢ - SQL External Routine Programming

Advanced SQL Engine
Teradata Database
Release Number
Release Date
July 2021
Content Type
Programming Reference
Publication ID
English (United States)

The type of intermediate results that an aggregate function needs to save depends on the type of calculation that the function performs.

Consider a standard deviation function STD_DEV(x) that uses the following equation:

Based on the calculation, the function needs to store the following:
  • N
  • sum(X 2)
  • sum(X)

The function can declare a C structure with components that match the necessary intermediate values:

typedef struct agr_storage {
    FLOAT n;
    FLOAT x_sq;
    FLOAT x_sum;
} AGR_Storage;

The function can then define a pointer to AGR_Storage that points to the FNC_Context_t.interim1 function argument.

Suppose the function declaration uses the following parameters:

void STD_DEV ( FNC_Phase     phase,
               FNC_Context_t *fctx,
               FLOAT         *x,
               FLOAT         *result,
               char          sqlstate[6] )

The following statement defines a pointer to AGR_Storage named s1 that points to the FNC_Context_t.interim1 function argument:

AGR_Storage *s1 = fctx->interim1;

The function can then easily access the storage area using the pointer to AGR_Storage. For example:

s1->n     = 0;
s1->x_sq  = 0;
s1->x_sum = 0;