Which attribute will be the most selective among: first name, last name, age, gender?

The most selective attribute from the list (first name, last name, age, gender) will be the one that has the greatest variety of values and the least repetition among the set of records.

• First name and last name can often repeat, especially if considering only one of them.
• Age is usually limited to a range (e.g., 0–120), but can be quite diverse.
• Gender typically has a very limited number of values (e.g., male, female).

Thus, the most selective will be the combination of first name and last name, as together they provide a more unique identification than each separately. If choosing a single attribute, then age is likely more selective than gender but less than first or last name.

In the context of indexes and queries, selectivity refers to the proportion of rows that satisfy the condition. The smaller the proportion, the higher the selectivity.