12c 新特性之PGA_AGGREGATE_LIMIT

11g

[root@roidb1 ~]# su - oracle$sqlplus / as sysdbaSQL*Plus: Release 11.2.0.4.0 Production on Wed Dec 13 21:40:51 2017Copyright (c) 1982, 2013, Oracle.  All rights reserved.Connected to:Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit ProductionWith the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,Data Mining and Real Application Testing optionsSQL> show parameter pga;NAME                                 TYPE        VALUE------------------------------------ ----------- ------------------------------pga_aggregate_target                 big integer 0SQL> 

12c

[root@rac01 ~]# su - oracleLast login: Fri Dec  8 16:09:27 CST 2017 on pts/0$sqlplus / as sysdbaSQL*Plus: Release 12.1.0.2.0 Production on Wed Dec 13 21:33:48 2017Copyright (c) 1982, 2014, Oracle.  All rights reserved.Connected to:Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit ProductionWith the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,Advanced Analytics and Real Application Testing optionsSYS@orcl1> show parameter pga;NAME                                 TYPE        VALUE------------------------------------ ----------- ------------------------------pga_aggregate_limit                  big integer 2Gpga_aggregate_target                 big integer 390MSYS@orcl1> 

--官方解释Property    DescriptionParameter typeBig integerSyntaxPGA_AGGREGATE_LIMIT = integer [K | M | G]Default valueBy default, PGA_AGGREGATE_LIMIT is set to the greater of 2 GB, 200% of PGA_AGGREGATE_TARGET, and 3 MB times the PROCESSES parameter. It will be set below 200% of PGA_AGGREGATE_TARGET if it is larger than 90% of the physical memory size minus the total SGA size, but not below 100% of PGA_AGGREGATE_TARGET.ModifiableALTER SYSTEMModifiable in a PDBNoRange of valuesOracle recommends that you do not set PGA_AGGREGATE_LIMIT below its default value. PGA_AGGREGATE_LIMIT cannot be set below its default value except in a text initialization parameter file (pfile) or binary server parameter file (spfile). If a value of 0 is specified, it means there is no limit to the aggregate PGA memory consumed by the instance.BasicNoPGA_AGGREGATE_LIMIT specifies a limit on the aggregate PGA memory consumed by the instance.There is no difference in behavior between PGA_AGGREGATE_LIMIT being explicitly set or being set to the default.

SYS@orcl1> show parameter pga;NAME                                 TYPE        VALUE------------------------------------ ----------- ------------------------------pga_aggregate_limit                  big integer 2Gpga_aggregate_target                 big integer 390MSYS@orcl1> alter system set pga_aggregate_limit=3G  --动态修改System altered.SYS@orcl1> show parameter pga;NAME                                 TYPE        VALUE------------------------------------ ----------- ------------------------------pga_aggregate_limit                  big integer 3Gpga_aggregate_target                 big integer 390MSYS@orcl1> alter system set pga_aggregate_limit=350m;alter system set pga_aggregate_limit=350m*ERROR at line 1:ORA-02097: parameter cannot be modified because specified value is invalidORA-00093: pga_aggregate_limit must be between 2048M and 100000G--大小不能小于2GSYS@orcl1> alter system set pga_aggregate_limit=2048m;System altered.SYS@orcl1> alter system set pga_aggregate_limit=2047m;alter system set pga_aggregate_limit=2047m*ERROR at line 1:ORA-02097: parameter cannot be modified because specified value is invalidORA-00093: pga_aggregate_limit must be between 2048M and 100000GSYS@orcl1> 

在 Oracle 12c R1 之前，没有选项可以用来限制和控制 PGA 的大小。虽然你设置某个大小为 PGA_AGGREGATE_TARGET 的初始参数，Oracle 会根据工作负载和需求来动态地增大或减小 PGA 的大小。而在 12c 中，你可以通过开启自动 PGA 管理来对 PGA 设定硬性限制，这需要对 PGA_AGGREGATE_LIMIT 参数进行设置。因此，你现在可以通过设置新的参数来对 PGA 设定硬性限制以避免过度使用 PGA。