The is set by default in Oracle 12c. The default value is the greater of the following. - 2 GB. - 200% of . It can be set below 200% if the is larger than 90% of (physical memory - SGA). The setting can't be below the . - 3 MB times the parameter.

The parameter can be set dynamically using the following command. Remember, it can't be set to a value lower than the parameter value. To revert to the pre-12c functionality, set the parameter to "0", which means the maximum PGA usage will no longer be managed.

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ALTER SYSTEM SET pga_aggregate_limit=6G SCOPE=BOTH;

ALTER SYSTEM SET pga_aggregate_limit=0 SCOPE=BOTH;

When the instance exceeds the setting, Oracle attempts to reduce PGA usage in the following way. - Sessions holding the most untunable memory have their calls aborted in an attempt to get below the PGA usage specified by the parameter. - If the instance PGA usage is still too big, the sessions holding the most untunable memory will be killed. SYS processes and job queue processes are not affected by this limit, but will write their PGA usage to trace files. Parallel queries will be treated like a single session, with their PGA usage summed. When a call or session is killed messages are directed to the client. In addition errors such as the following are written to the alert log. If the is not set high enough, your system may experience unexpected behaviour after an upgrade because of this. In addition, so people have reported high numbers of node evictions in RAC environments because of this. For more information see: - PGA_AGGREGATE_LIMIT Hope this helps. Regards Tim...

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Errors in file /u01/app/oracle/diag/rdbms/cdb1/cdb1/trace/cdb1_ora_53324.trc  (incident=66188):
ORA-04036: PGA memory used by the instance exceeds PGA_AGGREGATE_LIMIT