查看现在已有的SGA设置：
SQL> show parameter sga;
NAME TYPE VALUE
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 2G
sga_target big integer 2G更改SGA大小：
SQL> alter system set sga_max_size=4096M scope=spfile;
System altered.
/先备份spfile/
SQL> create pfile from spfile
SQL> alter system set sga_target=4096M scope=spfile;
System altered.如果是RAC环境，需要这样增加sid=’*’：
alter system set sga_target=20G scope=spfile sid=’*’;重启使更改生效：
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 4294967296 bytes
Fixed Size 2026296 bytes
Variable Size 805307592 bytes
Database Buffers 3472883712 bytes
Redo Buffers 14749696 bytes
Database mounted.
Database opened.
SQL> show parameter sga;
NAME TYPE VALUE
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 4G
sga_target big integer 4G如果更改spfile导致数据库无法启动，可以在pfile后填加附加参数，现在一般的pfile都是一行SPFILE=‘spfileSID.ora’，在后边加参数会覆盖spfile的已有参数：
SQL> alter system set lock_sga=true scope=spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORA-27102: out of memory
Linux-x86_64 Error: 12: Cannot allocate memory
[oracle@erpdevdb dbs]$ vi initerpstudydb.ora
SPFILE=’+DGSTUDY/erpstudydb/spfileerpstudydb.ora’
lock_sga=false
SQL> startup
ORACLE instance started.启动后，可以重新设置scope=spfile的参数，再删除pfile里后加的信息即可。
Windows系统可以用结束与开启系统服务的办法重启oracle系统进程，，记得要重启四个基本的服务项
