How does adding an index in Oracle take effect?

In Oracle, when you add an index, it takes immediate effect and can be queried right away. However, it is important to note that indexes only truly take effect after update operations (such as insert, update, delete).

After adding an index, you can ensure its effectiveness by performing the following actions:

1. Commit Transaction: If you have performed an insert operation on an index in a transaction, you must commit the transaction for the index to take effect.
2. Refresh buffer: Oracle uses a buffer to store data and indexes. If you add an index but do not refresh the buffer, the index may not take effect immediately. You can refresh the buffer by executing the following operation:
3. Use the ALTER SYSTEM FLUSH BUFFER_CACHE command to refresh the entire buffer cache.
4. Use the ALTER SYSTEM FLUSH SHARED_POOL command to refresh the shared pool, which includes the cache for data and indexes.
5. Restarting the database instance: If none of the above methods work, you can try restarting the database instance to ensure the effectiveness of the indexes.

It is important to note that in Oracle, the effectiveness of an index can also be influenced by other factors such as data distribution, query optimization, and the selection of indexes. Therefore, after adding an index, performance testing and optimization may be necessary to ensure the best use of the index.