How to address the issue of designated Oracle indexes not functioning properly.

If Oracle specified indexes are not effective, it may be due to the following reasons:

1. Outdated statistics: Outdated or inaccurate statistics of indexes may lead to the optimizer selecting incorrect execution plans. To resolve this issue, you can collect statistics of indexes by using the GATHER_INDEX_STATS procedure in the DBMS_STATS package.
2. Improper index selection: Sometimes, the optimizer may choose the wrong index to execute a query, resulting in inefficiency. You can use the /*+ INDEX(table index_name) */ hint to specify the optimizer to use a specific index.
3. Index failure: The index may have become invalid or corrupted, and needs to be recreated to resolve this issue.
4. The search criteria may not be suitable for indexing: Sometimes, the search criteria may not be suitable for indexing, resulting in ineffective indexing. You can check if the search criteria match the indexed fields and optimize them.
5. Fragmented index: Fragmented index may cause the index to become ineffective, you can use ALTER INDEX REBUILD to reconstruct the index.

If none of the above methods work to resolve the issue of ineffective indexing, you can try using the DBMS_ADVANCED_REWRITE package to manually specify the execution plan. Additionally, you may also consider redesigning the index or query statements to improve performance.