The magic of DUAL

问题

Under normal conditions (not using SYS or maybe using it)-

SQL> select * from dual;

D
-
X

Under not so normal conditions (connected as SYS)-

SQL> alter database close;
Statement processed.

SQL> select * from dual;
ADDR     INDX       INST_ID    D
-------- ---------- ---------- -
00FA6E50          0          1 X

I know DUAL is a special magic table (etc. etc.) but

What is different with DUAL when the DB is on standby? What is the relevance if ADDR, INDX, INST_ID in standby?

回答1:

Tom Kyte did an excellent job answering this question quite a few years ago, here:

http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:1562813956388

Hope that helps.

来源：https://stackoverflow.com/questions/12505049/the-magic-of-dual