How to Check Hidden Parameters in Oracle

In Oracle databases, many parameters are designed to be hidden from users to ensure that they don’t accidentally modify critical settings. These hidden parameters control various internal database features, optimizations, and behaviors that are not intended for routine administration. However, if you need to inspect or troubleshoot these parameters, Oracle provides a way to view and interact with them using specific SQL queries.

In this blog post, we’ll explore how to check hidden database parameters in Oracle, including how to view all hidden parameters and how to view a specific hidden parameter.

1. View All Hidden Parameters

Hidden parameters in Oracle are prefixed with an underscore (_). To view all hidden parameters, you can query the internal x$ksppi and x$ksppsv tables. These tables store parameter names and values respectively, and are not normally accessible through the standard V$PARAMETER or DBA_PARAMETERS views.

Here’s a SQL query you can use to view all hidden parameters:

SET PAGESIZE 50;
SET LINESIZE 150;
COL ksppinm FORMAT A40 HEADING 'Parameter Name' JUSTIFY CENTER;
COL ksppstvl FORMAT A50 HEADING 'Parameter Value' JUSTIFY CENTER;

SELECT 
    ksppinm, ksppstvl 
FROM 
    x$ksppi a, 
    x$ksppsv b 
WHERE 
    a.indx = b.indx 
    AND substr(ksppinm, 1, 1) = '_';

Explanation:

• ksppinm: This column contains the name of the hidden parameter.
• ksppstvl: This column contains the value of the hidden parameter.
• The query filters for parameters whose names start with an underscore (_), which are the hidden ones.

2. View a Specific Hidden Parameter

If you are looking for the value of a specific hidden parameter, you can modify the query to filter by the name of that parameter. For instance, if you’re interested in the hidden parameter _mv_refresh_shrink_log, you can use the following query:

SET PAGESIZE 50;
SET LINESIZE 150;
COL ksppinm FORMAT A40 HEADING 'Parameter Name' JUSTIFY CENTER;
COL ksppstvl FORMAT A50 HEADING 'Parameter Value' JUSTIFY CENTER;

SELECT 
  ksppinm, 
  ksppstvl 
FROM 
  x$ksppi a, 
  x$ksppsv b 
WHERE 
  a.indx = b.indx 
  AND ksppinm = '_mv_refresh_shrink_log';

• Explanation:
  • ksppinm: This is the name of the hidden parameter you’re querying.
  • ksppstvl: This gives the value of the hidden parameter you want to inspect.

By running this query, you can find out the current value of the specific hidden parameter _mv_refresh_shrink_log.

3. Why Check Hidden Parameters?

Hidden parameters often control advanced features or internal workings of the database. They are not typically exposed to users because they can have a significant impact on the performance or stability of the system if altered. However, in certain situations, such as debugging performance issues, troubleshooting internal processes, or tuning database features, you might need to examine these hidden parameters.

4. Caution While Modifying Hidden Parameters

While Oracle allows access to these hidden parameters for inspection, modifying them directly can be risky. Hidden parameters are not documented and can change between versions, so modifying them without understanding their purpose could lead to unexpected behavior or database instability.

If you need to change a hidden parameter, it’s highly recommended that you:

• Carefully review Oracle’s documentation or consult Oracle Support to understand the parameter’s function.
• Test any changes in a non-production environment before applying them to production.
• Ensure that you have a full backup and recovery plan in place before making any changes.

Conclusion

Checking hidden database parameters in Oracle is a powerful way to understand the internal workings of the database and troubleshoot advanced issues. By querying the x$ksppi and x$ksppsv tables, you can gain visibility into hidden parameters and their current values. However, always proceed with caution when considering modifying these parameters, as they can have a significant impact on the database’s behavior and performance.

Use the queries provided above to view hidden parameters and inspect their values as needed. If you’re not sure about a particular hidden parameter, it’s always a good idea to consult with Oracle Support or review the official Oracle documentation before making any change