What does the data structure with two or more columns, which are indexed, look like?

A data structure with an index on two or more columns is usually a composite (or concatenated) index. In a database, this is an index that considers the values of multiple fields together.

For example, if there is a table with fields (firstName, lastName, birthDate), you can create an index on (lastName, firstName). Such an index allows efficient searching of records by last name and first name simultaneously.

In a data structure, this might look like a tree or a hash table, where the key is a tuple of multiple values. For example:

class CompositeKey {
    String lastName;
    String firstName;

    // override equals and hashCode considering both fields
}

Map<CompositeKey, Person> index;

This index speeds up searches based on a combination of columns but is not always efficient for searching by a single column from the index.