Checking and Setting the NullBitVector - Analytics Database - Teradata Vantage

A NullBitVector identifies which elements in an ARRAY are present and not NULL, or present but is set to NULL.

A NullBitVector is an array of bytes that contain the presence bit for each initialized element in an ARRAY. Each byte contains 8 presence bits. The bits are stored in row-major order and are numbered from 0. A bit value of 1 means that the corresponding ARRAY element contains a non-null value. A bit value of 0 means that the ARRAY element is present but set to NULL.

The data type used to access the NullBitVector is defined in sqltypes_td.h as:

typedef unsigned char NullBitVecType;

Teradata provides the following library functions to check and set the bits of a NullBitVector.

The following shows a typical usage of a NullBitVector:

1. Call FNC_GetArrayTypeInfo to find out the number of elements in the ARRAY argument. Alternatively, you can call FNC_GetArrayElementCount to find out the total number of elements that are currently present in the ARRAY argument.
2. Allocate a new NullBitVector array:

   NullBitVector = (NullBitVecType*)FNC_malloc(nullVecBufSize);

   where nullVecBufSize is based on the number of elements in the ARRAY.

3. Initialize the NullBitVector by setting all bits to 0 (not present):

   memset(NullBitVector, 0, nullVecBufSize);

4. Call FNC_GetArrayElements to get the values of one or more ARRAY elements, passing the NullBitVector to the function. FNC_GetArrayElements modifies the NullBitVector to indicate which of the requested elements are present and not NULL, or present but set to NULL.
5. Call FNC_CheckNullBitVector or FNC_CheckNullBitVectorByElemIndex with the modified NullBitVector to interpret the results returned by FNC_GetArrayElements.
6. Call FNC_free to release allocated resources once you have processed the data.

For detailed information about these functions, see C Library Functions.