Decimal multiplication

For decimal multiplication, the precision and scale of the result depends on the precision and scale of the operands.

For multiplication, the precision of the result is MIN(n,p+p'), and the scale is MIN(n,s+s'). n is 31 if DEC31 is in effect or if the precision of at least one operand is greater than 15. Otherwise, n is 15.

If both operands have a precision greater than 15, the operation is performed using a temporary copy of the operand with the smaller precision. If the operands have the same precision, the second operand is selected. If more than 15 significant digits are needed for the integral part of the copy, the statement's execution is ended and an error occurs. Otherwise, the copy is converted to a number with precision 15, by truncating the copy on the right. The truncated copy has a scale of MAX(0,S-(P-15)), where P and S are the original precision and scale. If, in the process of truncation, one or more nonzero digits are removed, SQLWARN7 in SQLCA is set to W, indicating loss of precision.

When both operands have a precision greater than 15, the foregoing formulas for the precision and scale of the result still apply, with one change: for the operand selected as the copy, use the precision and scale of the truncated copy; that is, use 15 as the precision and MAX(0,S-(P-15)) for the scale.

Let n denote the value of the operand with the greater precision or the first operand in the case of operands with the same precision. The number of leading zeros in a 31-digit representation of n must be greater than the precision of the other operand. This is always the case if the precision of the operand is 15 or less. With greater precisions, overflow can occur even if the precision of the result is less than 31. For example, the expression: 
   10000000000000000000000000. * 1
will cause overflow because the number of leading zeros in the 31-digit representation of the large number and the precision of the small number are both 5 (see Arithmetic with an integer and a decimal operand).