The built in function FORMAT is used to display a number with group separators for the locale.
When the number is a very large string containing digits, the string is first converted to a double, then formatted. Both Server and Columnstore do it this way. However, the final output differs by quite a bit. This is because Server has their own double to string function my_fcvt() while columnstore uses snprintf(), which appears to have more rounding error.
For server, see item_strfunc.cc – String *Item_func_format::val_str_ascii(String *str)
For Columnstore see func_math.cpp – string Func_format::getStrVal()
We should either copy my_fcvt or find another way to eliminate the rounding error.