How to Check the Size of SGA and PGA in Oracle (And What to Do If Both Are 0)

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When working with an Oracle database, monitoring memory usage is crucial for performance tuning and troubleshooting. The System Global Area (SGA) and Program Global Area (PGA) are two key memory structures in Oracle. This guide will show you how to check their sizes and what steps to take if both values return as 0.

Checking the Size of SGA and PGA

To check the current size of the SGA and PGA, run the following queries:

1. Check SGA Size

SELECT name, value FROM v$parameter WHERE name = 'sga_target';

Alternatively, you can check total SGA memory allocation:

SELECT * FROM v$sga;

2. Check PGA Size

SELECT name, value FROM v$parameter WHERE name = 'pga_aggregate_target';

To get detailed PGA usage:

SELECT * FROM v$pgastat;

What to Do If Both SGA and PGA Show 0

If both values return 0, the database might not be properly initialized, or memory settings might be misconfigured. Here are the next steps:

1. Check If the Database Is Running

Run:

SELECT instance_name, status FROM v$instance;

If the status is not “OPEN,” the database might be in the wrong state.

2. Verify Memory Parameters in spfile or pfile

Check if memory parameters are explicitly set:

SHOW PARAMETER sga;
SHOW PARAMETER pga;

If these return 0, you might need to adjust them in the initialization parameter file (spfile or pfile) before restarting the database.

3. Check If Automatic Memory Management (AMM) Is Enabled

SELECT name, value FROM v$parameter WHERE name IN ('memory_target', 'memory_max_target');

If memory_target is set, Oracle automatically manages memory distribution.

4. Restart the Database with Proper Memory Allocation

If memory parameters are set to 0, update them in the spfile and restart the database:

ALTER SYSTEM SET sga_target = 2G SCOPE = SPFILE;
ALTER SYSTEM SET pga_aggregate_target = 1G SCOPE = SPFILE;
SHUTDOWN IMMEDIATE;
STARTUP;

Ensure values align with your system’s available memory.

5. Check for Errors in Alert Log

Review the Oracle alert log for errors related to memory allocation:

tail -100f $ORACLE_BASE/diag/rdbms/<DB_NAME>/<INSTANCE_NAME>/trace/alert_<INSTANCE_NAME>.log

Conclusion

Monitoring and configuring SGA and PGA are essential for Oracle performance. If both show 0, verify the database status, check memory parameters, and adjust settings accordingly. Restarting the database with proper memory allocation usually resolves the issue.

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