several questions about source configuration. Assume that the configured source has the following

sql_query = \
    SELECT id, group_id, UNIX_TIMESTAMP(date_added) AS date_added, \
        title, content \
    FROM documents

1) all non-full-text search fields after select must correspond to sql_attr_xxx = field_name ?
2) with regard to sql_attr_string, if it is written that this field will not be indexed, it will be saved, then this use is
3) is the index table structure self-defined sql_attr_string or sql_field_string