In Oracle database administration, how can you rebuild an existing index to improve performance or recover from corruption?

Introduction / Context:
Indexes in a relational database system such as Oracle can become fragmented, unbalanced, or sub optimal over time because of heavy insert, update, and delete activity. In some cases the index structure might also be damaged due to hardware or software issues. Database administrators therefore need a method to rebuild an index without dropping and manually recreating it from scratch. This question tests your understanding of the standard Oracle command used to rebuild an existing index while keeping its name and logical definition the same.

Given Data / Assumptions:

• An index already exists on one or more columns of a table.
• The index has become fragmented or needs to be rebuilt for performance or maintenance reasons.
• We want to rebuild the index without changing the indexed columns.
• The question is about Oracle style index maintenance using Data Definition Language.

Concept / Approach:
Oracle provides the ALTER INDEX statement to perform maintenance operations on existing indexes. One of the most common operations is REBUILD. The syntax is typically ALTER INDEX index_name REBUILD [ONLINE] [TABLESPACE new_ts]. This command recreates the index from the base table data, building a new balanced structure while keeping the index name and definition. It can also be used to move the index to a different tablespace or to change storage parameters. Using REBUILD is distinct from DROP INDEX plus CREATE INDEX, and it is designed to be more convenient and sometimes less disruptive, especially when used with ONLINE options in appropriate editions.

Step-by-Step Solution:
Step 1: Identify the index that is fragmented or needs maintenance by analyzing index statistics and performance. Step 2: Plan the maintenance window, because rebuilding an index can be resource intensive. Step 3: Issue the command ALTER INDEX index_name REBUILD; optionally include ONLINE or TABLESPACE clauses if needed. Step 4: Monitor the operation, check for errors, and ensure that dependent queries are still functioning as expected. Step 5: Re gather statistics after the rebuild to ensure that the cost based optimizer has accurate information about the new index structure.

Verification / Alternative check:
After the rebuild, the administrator can verify that the index exists and has a refreshed structure by querying data dictionary views such as DBA_INDEXES and DBA_SEGMENTS. Comparing clustering factors, leaf block counts, and used space before and after the rebuild can confirm that fragmentation was reduced. Alternative approaches include dropping and recreating the index, but that usually requires additional scripting and might cause longer downtime compared to an ALTER INDEX REBUILD operation.

Why Other Options Are Wrong:
Option B is wrong because DROP INDEX simply removes the index and does not rebuild it automatically. Option C is incorrect because ANALYZE INDEX VALIDATE STRUCTURE checks or analyzes the structure but does not rebuild it. Option D is misleading: ALTER TABLE MOVE can cause related indexes to become unusable, but it does not itself rebuild them. Option E is wrong because TRUNCATE TABLE removes data and affects the index contents, not the index structure in the sense of a maintenance rebuild.

Common Pitfalls:
A common mistake is thinking that any ANALYZE or statistics gathering operation rebuilds an index, which is not true. Another pitfall is rebuilding indexes too frequently without measuring actual performance issues, which wastes system resources. Administrators should also be careful when rebuilding very large indexes during peak hours, because this can increase I O load and impact users. Understanding the proper use of ALTER INDEX REBUILD is important for stable, performance aware database operations.

Final Answer:
The correct operation is to use ALTER INDEX index_name REBUILD to reconstruct and refresh the existing index structure.