Explain Two-Column Custom Indexes in Salesforce.

Two-column custom indexes are a specialized feature of the Salesforce platform. They are useful for list views and other situations in which you want to use one field to select the records to display and a second field to sort those records.

Two-column indexes are subject to the same restrictions as single-column indexes, with one exception. Two-column indexes can have nulls in the second column by default, whereas single-column indexes cannot, unless salesforce.com Customer Support has explicitly enabled the option to include nulls