游标怎么来？是什么样子？数据怎么fetch？

oracle游标探讨：
http://www.itpub.net/thread-1609083-1-1.html
asktom: Where does the Cursor stores the rows ?
https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:213366500346264333

docs oracle: Memory Architecture
http://docs.oracle.com/cloud/latest/db112/CNCPT/memory.htm#CNCPT1237