When does an index degrade performance?

An index degrades performance in the following cases:

• During frequent insert, update, and delete operations: each index needs to be maintained up-to-date, which adds overhead to write operations.
• If the index is not selective: for example, an index on a column with a small number of unique values (such as gender or status), the DBMS may ignore the index, and maintaining it will only consume resources.
• When there are too many indexes: this increases the amount of memory used and the time required to update data.
• When queries do not use the index: if queries do not filter or sort by indexed columns, the index becomes useless.

Thus, an index is useful when it speeds up queries, but it can slow down write operations and occupy extra space.