Persisted computed column with subquery

I have something like this

create function Answers_Index(@id int, @questionID int)
returns int
as begin
    return (select count([ID]) from [Answers] where [ID] < @id and [ID_Question] = @questionID)
end
go

create table Answers
(
    [ID] int not null identity(1, 1),
    [ID_Question] int not null,
    [Text] nvarchar(100) not null,
    [Index] as [dbo].[Answers_Index]([ID], [ID_Question]),
)
go

insert into Answers ([ID_Question], [Text]) values
    (1, '1: first'),
    (2, '2: first'),
    (1, '1: second'),
    (2, '2: second'),
    (2, '2: third')

select * from [Answers]

Which works great, however it tends to slow down queries quite a bit. How can I make column Index persisted? I have tried following:

create table Answers
(
    [ID] int not null identity(1, 1),
    [ID_Question] int not null,
    [Text] nvarchar(100) not null,
)
go

create function Answers_Index(@id int, @questionID int)
returns int
with schemabinding
as begin
    return (select count([ID]) from [dbo].[Answers] where [ID] < @id and [ID_Question] = @questionID)
end
go

alter table Answers add [Index] as [dbo].[Answers_Index]([ID], [ID_Question]) persisted
go

insert into Answers ([ID_Question], [Text]) values
    (1, '1: first'),
    (2, '2: first'),
    (1, '1: second'),
    (2, '2: second'),
    (2, '2: third')

select * from [Answers]

But that throws following error: Computed column 'Index' in table 'Answers' cannot be persisted because the column does user or system data access. Or should I just forget about it and use [Index] int not null default(0) and fill it in on insert trigger?

edit: thank you, final solution:

create trigger [TRG_Answers_Insert]
on [Answers]
for insert,开发者_如何学运维 update
as
    update [Answers] set [Index] = (select count([ID]) from [Answers] where [ID] < a.[ID] and [ID_Question] = a.[ID_Question])
        from [Answers] a 
        inner join [inserted] i on a.ID = i.ID      
go

You could change the column to be a normal column and then update its value when you INSERT/UPDATE that row using a trigger.

create table Answers
(
[ID] int not null identity(1, 1),
[ID_Question] int not null,
[Text] nvarchar(100) not null,
[Index] Int null
)

CREATE TRIGGER trgAnswersIU
ON Answers
FOR INSERT,UPDATE
AS 
   DECLARE @id int
   DECLARE @questionID int
   SELECT @id = inserted.ID, @questionID = inserted.ID_question


  UPDATE Answer a
  SET Index = (select count([ID]) from [Answers] where [ID] < @id and [ID_Question] = @questionID)
  WHERE a.ID = @id AND a.ID_question = @questionID

GO

NB* This is not fully correct as it wont work correctly on UPDATE as we wont have the "inserted" table to reference to get the ID and questionid. There is a way around this but i cant remember it right now :(

Checkout this for more info

Computed columns only store the formula of the calculation to perform. That is why it will be slower when querying the computed column from the table. If you want to persist the values to an actual table column, then you are correct about using a trigger.