In what cases can the use of indexes in SQL become ineffective or lead to their failure?

Indexes in SQL can become inefficient or stop working in the following cases:

• Frequent updates/inserts/deletes: Intensive write operations require constant index updates, which reduces performance.
• Inappropriate index type: For example, using an index on a column with high cardinality where most values are the same will not provide a benefit.
• Using functions or expressions in conditions: If a function is used on an indexable column in WHERE (e.g., WHERE LOWER(name) = 'abc'), the index may not be used.
• Too small or too large index: Indexes on very small tables may be redundant, while very large indexes can slow down operations.
• Lack of statistics or outdated statistics: The query optimizer may not choose an index due to incorrect data distribution information.
• Complex queries with many conditions: If the index does not cover all conditions, or the order of columns in a composite index does not match the conditions, the index may not be used.

Example: if you have an index on the "status" column, but the query filters using the function UPPER(status), the index will not be utilized.