我们知道从9i开始oracle提供了自动管理pga的功能，从此我们可以不管sort_area_size，hash_area_size等等烦人的设置，也不用再去计算繁琐的pga大小。但是怎么控制每个session使用pga的大小呢，会不会出现一个session用完所有pga，而其他session得不多空间的状况吗？
带着这个疑问我们可以来做一个实验。
实验的步骤很简单，初始pga_aggregate_target设定为100m，我们做一个排序的大查询，然后查看pga的使用空间。
然后修改pga_aggregate_target为1g，在做一个大查询，查看pga使用空间。
SQL> startup pfile=/test/inittest.ora
ORACLE instance started.

Total System Global Area 2710294504 bytes
Fixed Size 454632 bytes
Variable Size 805306368 bytes
Database Buffers 1903992832 bytes
Redo Buffers 540672 bytes
Database mounted.
Database opened.

SQL> show parameter pga

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
pga_aggregate_target big integer 100000000



SQL> conn test/test
Connected.
SQL> select name,value from v$statname a,v$mystat b where a.statistic#=b.statistic# and a.name like '%pga%';

NAME VALUE
---------------------------------------------------------------- ----------
session pga memory 243208
session pga memory max 243208
这里可以看到初始连接时pga使用了240k左右
SQL> set autotrace trace;


SQL> select title from test order by starts,ends,title,zoo,city;


2997531 rows selected.


Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=129273 Card=2932520
Bytes=173018680)

1 0 SORT (ORDER BY) (Cost=129273 Card=2932520 Bytes=173018680)
2 1 PARTITION RANGE (ALL)
3 2 TABLE ACCESS (FULL) OF 'TEST' (Cost=84738
Card=2932520 Bytes=173018680)





Statistics
----------------------------------------------------------
475 recursive calls
75 db block gets
930726 consistent gets
962505 physical reads
60 redo size
113681365 bytes sent via SQL*Net to client
2198688 bytes received via SQL*Net from client
199837 SQL*Net roundtrips to/from client
37 sorts (memory)
1 sorts (disk)
2997531 rows processed




SQL> set autotrace off;
SQL> select name,value from v$statname a,v$mystat b where a.statistic#=b.statistic# and a.name like '%pga%';

NAME VALUE
---------------------------------------------------------------- ----------
session pga memory 457496
session pga memory max 6355736

做完一个大查询，pga的使用峰值是达到了6m左右，这其中可能包含了一部分pga自己的开销

关闭数据库，修改pga_aggregate_target为1g，再重启数据库
SQL> conn / as sysdba
Connected.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> exit

[oracle@wanghai4 oracle]$ sqlplus "/as sysdba"

SQL*Plus: Release 9.2.0.4.0 - Production _disibledevent=173018680)





Statistics
----------------------------------------------------------
3740 recursive calls
28 db block gets
931577 consistent gets
955856 physical reads
60 redo size
113681365 bytes sent via SQL*Net to client
2198688 bytes received via SQL*Net from client
199837 SQL*Net roundtrips to/from client