Python 什么是'；额外'；在这个django代码中是什么意思

我不知道这是什么意思

谢谢

这是一个好主意

TOPIC_COUNT_SQL = """
SELECT COUNT(*)
FROM topics_topic
WHERE
    topics_topic.object_id = maps_map.id AND
    topics_topic.content_type_id = %s
"""
MEMBER_COUNT_SQL = """
SELECT COUNT(*)
FROM maps_map_members
WHERE maps_map_members.map_id = maps_map.id
"""

maps = maps.extra(select=SortedDict([
    ('member_count', MEMBER_COUNT_SQL),
    ('topic_count', TOPIC_COUNT_SQL),
]), select_params=(content_type.id,))