Function duckdb_appender_add_column 

pub unsafe extern "C" fn duckdb_appender_add_column(
    appender: duckdb_appender,
    name: *const c_char,
) -> duckdb_state

Appends a column to the active column list of the appender. Immediately flushes all previous data.

The active column list specifies all columns that are expected when flushing the data. Any non-active columns are filled with their default values, or NULL.

@param appender The appender to add the column to. @return DuckDBSuccess on success or DuckDBError on failure.