What are the reasons for Oracle indexes not being effective?

1. The index is too sparse: When most of the values in the index column are either the same or very rare, the effectiveness of the index will greatly decrease since the amount of data that needs to be retrieved during a query becomes very high.
2. Uneven data distribution: If the data in the indexed column is extremely unevenly distributed, for example, with some values appearing very frequently while others hardly appear at all, then the effectiveness of the index will decrease.
3. Large amount of data: When the data size is very large, the effectiveness of indexing may decrease. This is because indexing requires additional storage space and maintaining indexes during data insertion, retrieval, and deletion can lead to performance degradation.
4. Choosing an inappropriate index column can also result in the index not being effective. If the selected index column is rarely used or not relevant to the query conditions, the effectiveness of the index will be greatly reduced.
5. The search criteria are not appropriate: if the criteria include functions, operators, or type conversions, the index may not be utilized. Additionally, complex or multiple criteria combinations may also result in the index not being effective.
6. Creating too many indexes in a table can decrease the effectiveness of indexes because each index takes up storage space and maintaining them during data insertion, updating, and deletion can hinder performance.
7. Inaccurate database statistics can lead to ineffective query plans, as the database relies on these statistics to choose the most appropriate plan. If the statistics are inaccurate or outdated, the database may select inappropriate query plans, resulting in ineffective indexing.
8. Implicit type conversion: If the data type in the query criteria does not match the data type of the index column, the index may not be used. The database may perform implicit type conversion, but this can result in a decrease in query efficiency.
9. Database version issue: Some database versions may have bugs or limitations that prevent indexes from functioning properly, so it is important to pay attention to the database version and patches.

In conclusion, there are many reasons why an index may not be effective, and it typically requires a combination of specific analysis and optimization.