mysql 乘法精度

void Item_func_mul::result_precision()
{
/* Integer operations keep unsigned_flag if one of arguments is unsigned */
if (result_type() == INT_RESULT)
unsigned_flag= args[0]->unsigned_flag | args[1]->unsigned_flag;
else
unsigned_flag= args[0]->unsigned_flag & args[1]->unsigned_flag;
decimals= min(args[0]->decimals + args[1]->decimals, DECIMAL_MAX_SCALE);
uint est_prec = args[0]->decimal_precision() + args[1]->decimal_precision();
uint precision= min(est_prec, DECIMAL_MAX_PRECISION);
max_length= my_decimal_precision_to_length_no_truncation(precision, decimals,
unsigned_flag);
}

分析上面代码发现：

精度scale：
decimals= min(args[0]->decimals + args[1]->decimals, DECIMAL_MAX_SCALE);

精度precision：
uint est_prec = args[0]->decimal_precision() + args[1]->decimal_precision();
uint precision= min(est_prec, DECIMAL_MAX_PRECISION);