PyMySQL如何一般执行SELECT * FROM BIG_TABLE语句一边fetch数据呢？

gistfile1.py

conn = pymysql.connect(user='root',host='127.0.0.1', port=3306, db='test')
try:
    cursor = conn.cursor(pymysql.cursors.SSCursor)
    cursor.execute('select * from sbtest') 
    while True:
        row = cursor.fetchone()
        if not row:
            break
        print row
finally:
    cursor.close()