Will the optimizer always use the created index?

The query optimizer does not always use the created index. This depends on several factors:

• Data statistics: if the optimizer considers that using the index is inefficient (for example, if the index covers too many rows), it may choose a full table scan.
• Type of query: some operations (such as aggregations, sorting) may not use the index if it does not speed up execution.
• Index composition and query conditions: if the query conditions do not match the index or the index does not cover the necessary columns, the optimizer may refuse to use it.

Example: if an index is created on the age column, but the query filters by name, the index will not be used.

Thus, the presence of an index does not guarantee its use — the optimizer chooses the most efficient execution plan.