In what situations might using indexes in a database not provide the expected search acceleration?

Using indexes in a database may not speed up searches in the following situations:

• When queries return a large percentage of table rows — in this case, the DBMS may choose a full table scan as faster than using an index.
• If the index does not cover all fields used in the query, requiring additional data reads from the table.
• During frequent insert, update, or delete operations, indexes slow down these operations, which can negatively impact overall performance.
• If the index is created on a column with low selectivity (few unique values), it provides little help.

For example, if you have an index on the "status" field with two possible values (active/inactive), a query on this field may not use the index effectively.