What actions would you take if a database query does not use an index to find the required record?

If a database query does not use an index to find the required record, I would take the following steps:

1. Check the query execution plan (EXPLAIN) to understand why the index is not being used.
2. Ensure that the index actually exists and is suitable for the query conditions.
3. Verify that the conditions in the WHERE clause match the indexed columns (for example, no transformations or functions that prevent index usage).
4. If necessary, add or modify the index to cover the needed fields.
5. Optimize the query itself to make it more "index-friendly".

For example, if there is an index on the user_id column, but the query uses WHERE LOWER(user_id) = 'abc', the index will not be used. It is better to rewrite the query or add an index with a function.