In Oracle, the PLAN HASH value is a unique identifier used to represent a specific execution plan for a SQL statement. It is calculated based on the contents of the execution plan and is used by the query optimizer to determine if a cached plan can be reused. Changing the PLAN HASH value in Oracle can be useful in situations where you want to force Oracle to generate a new execution plan for a particular SQL statement.
One way to change the PLAN HASH value in Oracle is to use the DBMS shared pool package. By altering the SQL statement slightly, you can force Oracle to recalculate the PLAN HASH value and generate a new execution plan. Here’s how you can do it:
1. Query the current PLAN HASH value for the SQL statement you want to change:
“`sql
SELECT PHV, ADDRESS
FROM V$SQL
WHERE SQL_ID = ‘your_sql_id’;
“`
2. Use the DBMS shared pool package to flush the SQL statement from the shared pool:
“`sql
DECLARE
l_address VARCHAR2(16);
BEGIN
SELECT ADDRESS INTO l_address
FROM V$SQL
WHERE SQL_ID = ‘your_sql_id’;
DBMS_SHARED_POOL.PURGE(l_address, ‘C’);
END;
“`
3. Execute the SQL statement again to force Oracle to generate a new execution plan:
“`sql
SELECT * FROM your_table WHERE your_condition;
“`
4. Query the new PLAN HASH value to verify that it has changed:
“`sql
SELECT PHV
FROM V$SQL
WHERE SQL_ID = ‘your_sql_id’;
“`
By following these steps, you can effectively change the PLAN HASH value in Oracle and force the query optimizer to generate a new execution plan for a SQL statement.
Other Frequently Asked Questions:
1. Why would you want to change the PLAN HASH value in Oracle?
Changing the PLAN HASH value can be useful if you want to force Oracle to generate a new execution plan for a SQL statement that is not performing well with the current plan.
2. Can changing the PLAN HASH value improve query performance?
Yes, changing the PLAN HASH value can potentially improve query performance by allowing Oracle to generate a more efficient execution plan for the SQL statement.
3. Are there any risks associated with changing the PLAN HASH value?
While changing the PLAN HASH value can improve performance, it can also lead to suboptimal execution plans if not done carefully. It is important to test the new plan thoroughly before implementing it in a production environment.
4. Is there a way to change the PLAN HASH value without altering the SQL statement?
No, the PLAN HASH value is calculated based on the contents of the execution plan, so changing the SQL statement is necessary to change the PLAN HASH value.
5. How often should you change the PLAN HASH value?
It is recommended to change the PLAN HASH value only when necessary, such as when a SQL statement is not performing well with the current execution plan.
6. Can you change the PLAN HASH value for system-generated SQL statements?
No, the PLAN HASH value for system-generated SQL statements cannot be changed manually as Oracle manages these internally.
7. What are some alternative ways to improve query performance in Oracle?
Some alternative ways to improve query performance in Oracle include creating indexes, optimizing SQL statements, using hints, and gathering statistics.
8. Can the PLAN HASH value be changed for a specific user in Oracle?
No, the PLAN HASH value is tied to the SQL statement itself and not to a specific user in Oracle.
9. Is there a limit to how many times you can change the PLAN HASH value for a SQL statement?
There is no specific limit to how many times you can change the PLAN HASH value, but it is recommended to do so judiciously to avoid generating suboptimal execution plans.
10. Can changing the PLAN HASH value impact other SQL statements in Oracle?
Changing the PLAN HASH value for a specific SQL statement should not impact other SQL statements in Oracle as each statement has its own unique PLAN HASH value.
11. What role does the PLAN_HASH_VALUE column play in Oracle?
The PLAN_HASH_VALUE column in Oracle is used to identify the execution plan for a SQL statement and is used by the query optimizer to determine plan reusability.
12. Is it possible to change the PLAN HASH value for a stored procedure in Oracle?
No, the PLAN HASH value is associated with individual SQL statements and cannot be changed for a stored procedure as a whole.