SPI_fnumber returns the column number for the
column with the specified name.
If colname refers to a system column (e.g., oid) then the appropriate negative column number will be returned. The caller should be careful to test the return value for exact equality to SPI_ERROR_NOATTRIBUTE to detect an error; testing the result for less than or equal to 0 is not correct unless system columns should be rejected.
Column number (count starts at 1), or SPI_ERROR_NOATTRIBUTE if the named column was not found.