If an index is not used in a SELECT query, how do you determine the reason and what actions do you take?

If the index is not used in a SELECT query, I would take the following steps for diagnosis and resolution:

1. Check the execution plan (EXPLAIN) — see why the optimizer is not choosing the index. Perhaps the statistics are outdated or the query conditions do not suit index usage.

2. Analyze WHERE conditions — ensure that filtering conditions match the indexed columns and that there are no transformations (functions, operations) that prevent index usage.

3. Update statistics — run ANALYZE TABLE or similar to provide the optimizer with current data.

4. Check index selectivity — if the index covers a column with low selectivity, the optimizer might prefer a full scan.

5. Check index structure — the index might not be suitable for the current query (e.g., column order in a composite index).

6. Force index usage — temporarily use a hint (if supported by the DBMS) to test the impact of the index.

7. Optimize the query — rewrite the query to better fit the index.

Typically, the reason is that the optimizer considers index usage less efficient, and the task is to help it make the right decision through analysis and adjustment of the query or statistics.