Suggest an index for the users table with fields gender and age if you need to select users by gender or age.

For filtering users by gender or age, it is optimal to create two separate indexes, as queries may filter by one of these fields independently:

CREATE INDEX idx_users_gender ON users(gender);
CREATE INDEX idx_users_age ON users(age);

If queries often filter by both fields simultaneously, a composite index can be added:

CREATE INDEX idx_users_gender_age ON users(gender, age);

However, for filtering by a single field, a composite index will be less efficient than separate indexes. Therefore, in most cases, it is better to use two separate indexes.