当会话退出或者用户提交commit和回滚rollback事务的时候，临时表的数据自动清空，但是临时表的结构以及元数据还存储在用户的数据字典中。
临时表的定义对所有会话SESSION都是可见的,但是表中的数据只对当前的会话或者事务有效.
临时表经常被用于存放一个操作的中间数据（数据处理的中间环节）。
临时表由于不产生redo，能够提高数据操作的性能。
临时表不会产生锁和等待。
实验如下：
12:35:26 SQL> create global temporary table temp1 on commit delete rows as select * from dba_extents;
Table created
14:06:29 SQL> select count(*) from temp1;
COUNT(*)
----------
0
14:08:16 SQL> insert into temp1 select * from dba_extents;
9221 rows inserted
14:09:02 SQL> select count(*) from temp1;
COUNT(*)
----------
9221
14:09:20 SQL> commit;
Commit complete
14:09:24 SQL> select count(*) from temp1;
COUNT(*)
----------
0
14:09:27 SQL> select * from temp1;
OWNER SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE TABLESPACE_NAME EXTENT_ID FILE_ID BLOCK_ID BYTES BLOCKS RELATIVE_FNO
------------------------------ -------------------------------------------------------------------------------- ------------------------------ ------------------ ------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
14:15:16 SQL> create global temporary table temp2 on commit preserve rows as select * from dba_extents;
Table created
14:15:39 SQL> select count(*) from temp2;
COUNT(*)
----------
9221
14:16:19 SQL> insert into temp2 select * from dba_extents;
9222 rows inserted
14:16:58 SQL> select count(*) from temp2;
COUNT(*)
----------
18443
14:17:15 SQL> commit;
Commit complete
14:17:18 SQL> select count(*) from temp2;
COUNT(*)
----------
18443
14:17:19 SQL>
再打开一个会话进行查询：
SQL> select count(*) from temp2;
COUNT(*)
----------
0
SQL> select * from temp2;
OWNER SEGMENT_NAME PARTITION_NAME SEGMENT_TYPE TABLESPACE_NAME EXTENT_ID FILE_ID BLOCK_ID BYTES BLOCKS RELATIVE_FNO
------------------------------ -------------------------------------------------------------------------------- ------------------------------ ------------------ ------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
SQL>