purge recyclebin之後dba_segments仍然有BIN$段

現象：
purge recyclebin之後dba_segments仍然有BIN$段。
如下，執行了purge recyclebin之後：
SQL> select segment_name,SEGMENT_TYPE from dba_segments where tablespace_name like 'USERS' and owner='ZHOU186'
 2 ;

SEGMENT_NAME SEGMENT_TYPE
--------------------------------------------------------------------------------- ------------------
BIN$xd87Y+adofPgQAB/AQB9yA==$0 TABLE
BIN$xd87Y+acofPgQAB/AQB9yA==$0 TABLE
ZHOU_WORK_UNITE_??? TABLE
ZHOU_WORK_UNITE_??? TABLE
ZHOU_WORK_UNITE_?? TABLE
BIN$0QhC65ubuMzgQAB/AQBaBg==$0 TABLE
ZHOU_WORK_UNITE_??? TABLE
ZHOU_WORK_UNITE_??? TABLE
BIN$wJ9k2G65qpDgQAB/AQAj+g==$0 TABLE
ZHOU_WORK_UNITE_??? TABLE
BIN$0QhC65uauMzgQAB/AQBaBg==$0 TABLE
ZHOU_WORK_UNITE_??? TABLE
BIN$0QhC65uduMzgQAB/AQBaBg==$0 TABLE
BIN$0QhC65ucuMzgQAB/AQBaBg==$0 TABLE
SQL> desc ZHOU186."BIN$xd87Y+adofPgQAB/AQB9yA==$0";
 Name Null? Type
 ----------------------------------------------------------------------------------------------------------------- -------- ----------------------------------------------------------------------------
 BEGIN_DATE VARCHAR2(10)
 BEGIN_TIME VARCHAR2(10)
 END_DATE VARCHAR2(10)
 END_TIME VARCHAR2(10)
 POSITION1 VARCHAR2(30)
 PRG_NUM VARCHAR2(30)
 WORK_TIME FLOAT(126)
 FIRST_CHECK NUMBER(38)
 PERSON_NUMBER VARCHAR2(20)
 JOB_BIN VARCHAR2(20)
 JOB_BINNUMBER NUMBER(38)

分析：
客戶的操作步驟大致是：
[oracle@MESZHOUDB ~]$ sqlplus / as sysdba

SQL*Plus: Release 10.2.0.4.0 - Production on Thu May 16 09:55:40 2013

Copyright (c) 1982, 2007, Oracle. All Rights Reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL> alter session set current_schema=ZHOU186;            <====注意到是set current_schema，並不是直接登入進來

Session altered.

SQL> purge recyclebin;

Recyclebin purged.

SQL>
SQL>
SQL> select segment_name from dba_segments where tablespace_name like 'USERS' and owner='ZHOU186'
 2 ;

SEGMENT_NAME
--------------------------------------------------------------------------------
BIN$xd87Y+adofPgQAB/AQB9yA==$0
BIN$xd87Y+acofPgQAB/AQB9yA==$0
ZHOU_WORK_UNITE_???
ZHOU_WORK_UNITE_???
ZHOU_WORK_UNITE_??
BIN$0QhC65ubuMzgQAB/AQBaBg==$0
ZHOU_WORK_UNITE_???
ZHOU_WORK_UNITE_???
BIN$wJ9k2G65qpDgQAB/AQAj+g==$0
ZHOU_WORK_UNITE_???
BIN$0QhC65uauMzgQAB/AQBaBg==$0

SEGMENT_NAME
--------------------------------------------------------------------------------
ZHOU_WORK_UNITE_???
BIN$0QhC65uduMzgQAB/AQBaBg==$0
BIN$0QhC65ucuMzgQAB/AQBaBg==$0
BIN$0QhC65ueuMzgQAB/AQBaBg==$0
ZHOU_WORK_UNITE_???
BIN$w2uuBw1tJa/gQAB/AQB7Eg==$0
BIN$w2uuBw1sJa/gQAB/AQB7Eg==$0
BIN$w2uuBw1uJa/gQAB/AQB7Eg==$0
BIN$w2uuBw1vJa/gQAB/AQB7Eg==$0


Oracle文檔的說明：
The CURRENT_SCHEMA parameter changes the current schema of the session to the specified schema. Subsequent unqualified references to schema objects during the session will resolve to objects in the specified schema.
This setting offers a convenient way to perform operations on objects in a schema other than that of the current user without having to qualify the objects with the schema name. This setting changes the current schema, but it does not change the session user or the current user, nor does it give the session user any additional system or object privileges for the session.

For example:

The table "T1" is owned by user "TEST" and user "SCOTT" doesn't have a table named "T1":

SQL>CONNECT TEST/TEST
SQL>GRANT SELECT ON T1 TO SCOTT;
SQL>CONNECT scott/tiger
SQL>SELECT * FROM SCOTT.T1; <<<<< you must specify the table prefix "SCOTT."
SQL>ALTER SESSION SET CURRENT_SCHEMA = test;
SQL>SELECT * FROM T1; <<<<< Now you need not to add the prefix "SCOTT."

After the session set CURRENT_SCHEMA = test, the only change is that the user "SCOTT" can access the table "TEST.T1" without specifying the table prefix.
The current user is still "TEST", This time when you perform a "purge recyclebin", it just purges the recyclebin in user "TEST", not "SCOTT".

For more details, please refer to:

Oracle? Database SQL Reference
10g Release 2 (10.2)
B14200-02

http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_2012.htm#SQLRF00901

一句話總結：都是基本概念沒過關，很多事情想當然的去理解，Oracle文檔還是值得很多人細細的讀。