Include Columns in Index : SQL Server

Question

I have one question regarding include column in the indexes. If I have an include column which is updating its value quite often, does it update the index every time it's updating the value or does the index only update when key columns are updating? Does include column updates add performance issues? According the example below, if the Balance and DiscountedAmount keep updating does it update the index IX_Temp_RegistrationNumber for every updates?

CREATE TABLE #Temp (
    TheID BIGINT
    ,RegistrationNumber BIGINT
    ,ClientName VARCHAR(50)
    ,Balance NUMERIC(10, 2)
    ,DiscountedAmount NUMERIC(10, 2)
    )

CREATE NONCLUSTERED INDEX [IX_Temp_RegistrationNumber] ON [#Temp] ([RegistrationNumber] ASC) INCLUDE (
    [Balance]
    ,[DiscountedAmount]
    )
GO 

Answer 1

Actually if you get Actual Plan, you will be able to see, indexes that modified by every update.

When you do update, Cluster Index and all indexes that keys and included column , had updated, will be modified.

Example 1 :
update #Temp set ClientName = 'foo'
Execution plan : The clustered index will be update

Example 2:
update #Temp set Balance = 'foo'
Execution plan : The clustered index and [IX_Temp_RegistrationNumber] will be update.

Tip :If your table haven't so many indexes or update operations isn't high, then there isn't any performance issue.