For some operators in Aster (for example, "int_col > '1000'") the optimizer will add a type conversion automatically (for example, "int_col > cast('1000' as integer)"A).
This conversion is not available in R.
Because Aster does not detect the potential conversions, which could indicate the available conversions, the translated SQL is put into Aster, and an error message at run-time is returned if the conversion is not available.