Oracle 11g默认使用AMM(Automatic Memory Management，自动内存管理)功能。在安装数据库过程中，指定Oracle使用内存的百分比，这个取值就作为MEMORY_TARGET和MEMORY_MAX_TARGET的初始取值使用。
Oracle版本
sql> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production PL/sql Release 11.2.0.1.0 - Production CORE 11.2.0.1.0 Production TNS for Linux: Version 11.2.0.1.0 - Production NLSRTL Version 11.2.0.1.0 - Production
查看MEMORY_TARGET和相关参数值
sql> show parameter memory NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ hi_shared_memory_address integer 0 memory_max_target big integer 2496M memory_target big integer 2496M shared_memory_address integer 0
sql> show parameter sga; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ lock_sga boolean FALSE pre_page_sga boolean FALSE sga_max_size big integer 2496M sga_target big integer 0
sql> show parameter pga; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ _pga_max_size big integer 209700K pga_aggregate_target big integer 0
修改MEMORY_TARGET
说明：通过alter system语句来修改MEMORY_TARGET。需要注意的是，MEMORY_TARGET是个动态参数，而MEMORY_MAX_TARGET是个静态参数。
当修改值<=MEMORY_MAX_TARGET时，可以随意修改MEMORY_TARGET值；当修改值>MEMORY_MAX_TARGET时，必须首先修改MEMORY_MAX_TARGET参数值，重启数据库使静态参数生效后再去修改MEMORY_TARGET值，否则，数据库会报错。
这里只介绍修改值<=MEMORY_MAX_TARGET情况。
sql> alter system set memory_max_target=26G scope=spfile; System altered.
sql> shutdown immediate Database closed. Database dismounted. ORACLE instance shut down.
sql> startup ORACLE instance started. Total System Global Area 2.7793E+10 bytes Fixed Size 2214096 bytes Variable Size 2.7380E+10 bytes Database Buffers 268435456 bytes Redo Buffers 141578240 bytes Database mounted. Database opened.
sql> show parameter memory; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ hi_shared_memory_address integer 0 memory_max_target big integer 26G memory_target big integer 2560M shared_memory_address integer 0
sql> alter system set memory_target=24G; System altered.
sql> show parameter memory; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ hi_shared_memory_address integer 0 memory_max_target big integer 26G memory_target big integer 24G shared_memory_address integer 0
sql> show parameter sga; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ lock_sga boolean FALSE pre_page_sga boolean FALSE sga_max_size big integer 26G sga_target big integer 0
