This routine creates a histogram value for a data element. Place the address of this routine in the dtd_helem_addr field of the IIADD_DT_DFN structure. If the II_DT_NOHISTOGRAM attribute is present, then this routine is not necessary.

A histogram value is a representation of the data element. The DBMS query optimizer uses histogram values in the evaluation of query plans. The optimizer restricts the length of the histogram value to 8.

The comparison of the histograms of two values must match the comparison of their respective values, because the histogram value definition (if a < b, then h(a) < h(b) ) assumes that 'a < b' uses the same compare routine. Histograms values have a type-for details, see the description hg_dtln Routine.