What are the reasons for Oracle index failure?

There are several common reasons that can cause Oracle indexes to become ineffective.

1. Uneven data distribution: If the data distribution in the index column is not balanced, meaning some values appear more frequently while others appear less frequently, the selectivity of the index will decrease, causing it to become ineffective.
2. Inaccurate statistics can lead to ineffective indexes as the optimizer relies on them to generate execution plans.
3. When the amount of data is small, using an index may lead to extra IO operations, resulting in decreased query performance. In this situation, Oracle may opt for a full table scan instead of using an index.
4. Operations on indexed columns may not be suitable for indexing: certain operations such as using functions, type conversions, or fuzzy queries may cause the index to be ineffective. In such cases, Oracle may choose not to use the index.
5. Having NULL values on the indexed column can cause a decrease in performance when using the index, as NULL values require additional storage space in the index.
6. Frequent data updates can lead to indexes becoming ineffective due to maintenance costs. In some cases, Oracle may opt for full table scans instead of using indexes.

It is important to note that the above are just some common reasons, and the actual situation may vary depending on the specific database design and query operation.