When I was reading a oracle document, it said that the select action will not put any locks on the table you are reading. That is different from what I thought before. I understand that if a table were selecting, the session will not put any locks to stop othere sesssion from writing the table. Or I should say I understand that read will not block write in Oracle. But I was also thinging that there should be a table lock protecting the structure of the table. This lock is not to protect the data of the table only the table structure. If there is not this kind of lock that means we can drop a table when we reading it. And I think that will make the reading session be aborted because the drop action will not generate UNDO information. So I did a test.
1. first i create a table below. Then I run a sqlstatement which is very time consuming
1 SQL> create table scott.emp1 as select * from scott.emp; 2 3 Table created. 4 5 Elapsed: 00:00:00.09 6 SQL> 7 SQL> select * from scott.emp1 where (select count(*) from dba_objects,dba_tables) = (select count(*) from dba_objects,dba_tables); 8 9 10 EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 11 ---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ---------- 12 7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20 13 7499 ALLEN SALESMAN 7698 1981-02-20 00:00:00 1600 300 30 14 7521 WARD SALESMAN 7698 1981-02-22 00:00:00 1250 500 30 15 7566 JONES MANAGER 7839 1981-04-02 00:00:00 1 20 16 7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 17 7698 BLAKE MANAGER 7839 1981-05-01 00:00:00 2850 30 18 7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10 19 7788 SCOTT ANALYST 7566 1987-04-19 00:00:00 3000 20 20 7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10 21 7844 TURNER SALESMAN 7698 1981-09-08 00:00:00 1500 0 30 22 7876 ADAMS CLERK 7788 1987-05-23 00:00:00 1100 20 23 24 EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 25 ---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ---------- 26 7900 JAMES CLERK 7698 1981-12-03 00:00:00 950 30 27 7902 FORD ANALYST 7566 1981-12-03 00:00:00 99999 20 28 7934 MILLER CLERK 7782 1982-01-23 00:00:00 9999 10 29 30 14 rows selected.
2. Before that select finish, I drop the table in another session.
SQL> drop table scott.emp1 purge; Table dropped.
3. But the select still can run successfully
1 SQL> select * from scott.emp1 where (select count(*) from dba_objects,dba_tables) = (select count(*) from dba_objects,dba_tables); 2 3 4 EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 5 ---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ---------- 6 7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20 7 7499 ALLEN SALESMAN 7698 1981-02-20 00:00:00 1600 300 30 8 7521 WARD SALESMAN 7698 1981-02-22 00:00:00 1250 500 30 9 7566 JONES MANAGER 7839 1981-04-02 00:00:00 1 20 10 7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 11 7698 BLAKE MANAGER 7839 1981-05-01 00:00:00 2850 30 12 7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10 13 7788 SCOTT ANALYST 7566 1987-04-19 00:00:00 3000 20 14 7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10 15 7844 TURNER SALESMAN 7698 1981-09-08 00:00:00 1500 0 30 16 7876 ADAMS CLERK 7788 1987-05-23 00:00:00 1100 20 17 18 EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 19 ---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ---------- 20 7900 JAMES CLERK 7698 1981-12-03 00:00:00 950 30 21 7902 FORD ANALYST 7566 1981-12-03 00:00:00 99999 20 22 7934 MILLER CLERK 7782 1982-01-23 00:00:00 9999 10 23 24 14 rows selected. 25 26 Elapsed: 00:01:04.59
There were two theorie
https://forums.oracle.com/forums/thread.jspa?messageID=10689825
http://www.itpub.net/forum.php?mod=viewthread&tid=1776743&page=1#pid21235290
This should be figure out some other time