Computed columns

Hi,

Consider the following example

create table sample

(col1 int,

col2 int ,

col3 AS col1 + col2) PERSISTED NOT NULL)

basically col3 is a computed column. Now when ever a row in col1 or col2 is updated the computed column will reflect the new value. how does this happen in the background. does this use row level triggers or what other mechanism is used to maintain col3 - computed column

the value does not exist by default

when you select a record and you included the calculated column

the server reevaluates everything which

is one of the great disadvantage of the computed column.

computation is being done all over again when you select from this column

|||I have marked it PERSISTED ...meaning that the column is saved in the database. I think what you are talking about is the computed column without using the PERSISTED key word.|||

sorry i wasn't aware of that new feature

any way

Their values are updated when any columns that are part of their calculation change

|||I would say magic :) Seriously, it would be done at a physical implementation level below what we have access to, much like values in an index get maintained. I think if you thought of it sort of like a row-level trigger it wouldn't be "wrong," but it is not through any mechanism that we have direct access to for sure.|||the reason why i posted the question is : If i use a lot of computed columns in my database, will it cause any kind of performance problems. so far what ever i have read about computed columns, no where it is mentioned that using computed columns may cause performance problems. Please let me know if you have any information.|||I think the question is more about how you need the data. There will be a performance hit, whether you persist them, or not. The difference will be based on whether you modify data more, or read it more. If it is a frequently used column (or might be) try persisting it. If it doesn't slow you down too much then that would be the best idea. Unless your formula is tremendously complex, I doubt you will even notice.|||

one good thing in answering in this forum is that you

got to learn new things. anyway here's what BOL has to say

Unless otherwise specified, computed columns are virtual columns that are not physically stored in the table. Their values are recalculated every time they are referenced in a query. The SQL Server 2005 Database Engine uses the PERSISTED keyword in the CREATE TABLE and ALTER TABLE statements to physically store computed columns in the table. Their values are updated when any columns that are part of their calculation change. By marking a computed column as PERSISTED, you can create an index on a computed column that is deterministic but not precise. Additionally, if a computed column references a CLR function, the Database Engine cannot verify whether the function is truly deterministic. In this case, the computed column must be PERSISTED so that indexes can be created on it.

In terms of performance therefore persisted columns, performs better than non-persisted. Except of course when we hard talking of harddisk consumption. persisted column may even outrun a column-trigger solution and the first is easier to maintain over the later.

|||Thanks for all your replies.