12c 新特性之PGA_AGGREGATE_LIMIT

11g

[root@roidb1 ~]# su - oracle <roidb1:orcl1:/home/oracle>$sqlplus / as sysdba SQL*Plus: Release 11.2.0.4.0 Production on Wed Dec 13 21:40:51 2017 Copyright (c) 1982, 2013, Oracle. All rights reserved. Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP, Data Mining and Real Application Testing options SQL> show parameter pga; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ pga_aggregate_target big integer 0 SQL> 

12c

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

--官方解释 Property Description Parameter type Big integer Syntax PGA_AGGREGATE_LIMIT = integer [K | M | G] Default value By 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. Modifiable ALTER SYSTEM Modifiable in a PDB No Range of values Oracle 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. Basic No PGA_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 2G pga_aggregate_target big integer 390M SYS@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 3G pga_aggregate_target big integer 390M SYS@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 invalid ORA-00093: pga_aggregate_limit must be between 2048M and 100000G --大小不能小于2G SYS@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 invalid ORA-00093: pga_aggregate_limit must be between 2048M and 100000G SYS@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。