降低SQLSET相关表WRI$_SQLSET_PLAN_LINES高水位线，释放SYSAUX表空间

客户提出SYSAUX空间太大，已经占据了20多G的空间，登陆系统发觉SYSAUX表空间中占据TOP SEGMENT的主要就是WRI$_SQLSET_PLAN_LINES表

SQL> select *
  2    from (select bytes / 1024 / 1024 / 1024, segment_name, owner, segment_type
  3            from dba_segments
  4           where tablespace_name = 'SYSAUX'
  5           order by bytes desc)
  6   where rownum < 20
  7  ;

BYTES/1024/1024/1024 SEGMENT_NAME                                                                     OWNER                          SEGMENT_TYPE
-------------------- -------------------------------------------------------------------------------- ------------------------------ ------------------
        3.9814453125 WRI$_SQLSET_PLAN_LINES                                                           SYS                            TABLE
        2.8681640625 WRH$_SQLTEXT                                                                     SYS                            TABLE
        2.3955078125 SYS_LOB0000009295C00038$$                                                        SYS                            LOBSEGMENT
        1.0615234375 WRH$_SYSMETRIC_HISTORY                                                           SYS                            TABLE
        0.9951171875 WRH$_SYSMETRIC_HISTORY_INDEX                                                     SYS                            INDEX
        0.8427734375 I_WRI$_OPTSTAT_H_OBJ#_ICOL#_ST                                                   SYS                            INDEX
        0.7587890625 SYS_LOB0000009002C00038$$                                                        SYS                            LOBSEGMENT
         0.751953125 WRH$_ACTIVE_SESSION_HISTORY                                                      SYS                            TABLE PARTITION
         0.673828125 WRH$_SQL_PLAN                                                                    SYS                            TABLE
         0.646484375 WRH$_ACTIVE_SESSION_HISTORY                                                      SYS                            TABLE PARTITION
         0.599609375 WRI$_OPTSTAT_HISTGRM_HISTORY                                                     SYS                            TABLE
         0.599609375 WRH$_ACTIVE_SESSION_HISTORY                                                      SYS                            TABLE PARTITION
        0.5849609375 WRH$_ACTIVE_SESSION_HISTORY                                                      SYS                            TABLE PARTITION
          0.55078125 WRH$_ACTIVE_SESSION_HISTORY                                                      SYS                            TABLE PARTITION
        0.5244140625 WRH$_ACTIVE_SESSION_HISTORY                                                      SYS                            TABLE PARTITION
        0.4052734375 I_WRI$_OPTSTAT_H_ST                                                              SYS                            INDEX
         0.380859375 WRH$_SYSMETRIC_SUMMARY                                                           SYS                            TABLE
        0.3115234375 WRI$_SQLSET_PLAN_LINES_PK                                                        SYS                            INDEX
        0.2900390625 WRI$_SQLSET_PLANS                                                                SYS                            TABLE

19 rows selected
对于WRI$_SQLSET_PLAN_LINES表查看其中数据只有0条，但是segment分配已经达到了4G多，这个优先考虑的就是在系统闲暇时间进行move或者shrink space操作：

SQL> alter table WRI$_SQLSET_PLAN_LINES shrink space;
alter table WRI$_SQLSET_PLAN_LINES shrink space
*
ERROR at line 1:
ORA-10662: Segment has long columns

SQL> !oerr ora 10662
10662, 00000, "Segment has long columns"
// *Cause:  Shrink was issued on a segment with long columns. This is not
//          supported.
// *Action:

SQL> alter table _SQLSET_PLAN_LINES
  2
SQL> alter table WRI$_SQLSET_PLAN_LINES move;
alter table WRI$_SQLSET_PLAN_LINES move
*
ERROR at line 1:
ORA-00997: illegal use of LONG datatype
而对于包含LONG的数据表无法进行move或者shrink space，oracle提供了一种方法，就是首先exp或者expdp将这部分数据导出，然后truncate该表，最后将数据导入到该表中。这里对于WRI$_SQLSET_PLAN_LINES表仅仅只是存储的SQL SET的信息，而这部分SQL SET已经被删除掉，这里直接truncate该表就可以释放出该表的空间，当然如果表中有数据则可以参考上面这种处理方式。

SQL中select与set的区别

).同时对多个变量同时赋值时

declare @a varchar(128), @b varchar(128)
SET @a='ABC',@b='EFG'
GO --报错：消息 102，级别 15，状态 1，第 3 行 ',' 附近有语法错误。

declare @a varchar(128), @b varchar(128)
SELECT @a='ABC',@b='EFG'
GO --正确运行

(2).表达式返回多个值时

在说明这一项前我们先创建一个要用到的表，并对其赋值，代码如下：

赋值测试表--fuzhiTEST
create table fuzhiTEST(
id int ,
name varchar(128)
)
go
insert into fuzhiTEST(id,name) values(1,'name1')
insert into fuzhiTEST(id,name) values(2,'name2')
insert into fuzhiTEST(id,name) values(3,'name3')
go

declare @name varchar(128)
set @name = (select name from fuzhiTEST)
GO  --报错：消息 512，级别 16，状态 1，第 2 行
                 子查询返回的值不止一个。当子查询跟随在 =、!=、<、<=、>、>= 之后，或子查询用作表达式时，这种情况是不允许的。

declare @name varchar(128)
select @name =name from fuzhiTEST
print @name
GO  --正确运行：显示 name3

(3).表达式未返回值时

declare @name varchar(128)
set @name = 'SET初始值'
set @name = (select name from fuzhiTEST where id = 4 )
print @name
GO   --正确运行：显示NULL

declare @name varchar(128)
set @name = 'SELECT初始值'
select @name = name from fuzhiTEST where id = 4
print @name --正确运行：显示 SELECT初始值
GO

注意：SELECT 也可以将标量子查询的值赋给变量，如果标量子查询不返回值，则变量被置为 null 值（此时与使用 SET 赋值是完全相同的）。
对标量子查询的概念大家可能觉得陌生，下面举个例子说明下（对比上面绿色部分）：

declare @name varchar(128)
set @name = 'SELECT初始值'
select @name =(select name from fuzhiTEST where id = 4 )
print @name --正确运行：显示NULL
GO