Before trying to understand what "an index on a Computed Column" is, it is good to have a basic understanding of what a Computed Column is.
As per MSDN, a Computed Column is computed from an expression that can use other columns in the same table. The expression can be a non-computed column name, constant, function, or any combination of these, connected by one or more operators. The expression cannot be a subquery.
By default, a Computed Column is a virtual column and it is recalculated every time we call it, until we specify it as
PERSISTED in the
CREATE TABLE or
ALTER TABLE commands.
If a Computed Column is defined as being
PERSISTED, it stores the calculated value and those stored values are ...