Computed Columns

Question

Post reply

Computed Columns

ningaraju.n

SSCrazy

Points: 2322
More actions
November 10, 2010 at 9:25 pm

#231228

Hi Guys,
Is there a way to reference columns of other table in computed columns except triggers?
Thanks in Advance

Viewing 9 posts - 1 through 8 (of 8 total)

You must be logged in to reply to this topic. Login to reply

Johan Bijnens SSC Guru Points: 135369 More actions · Answer 1

I don't think so. simple reason ... this cannot be "at run time" but must be persisted, so a trigger is the only way.

Johan

Learn to play, play to learn !

Dont drive faster than your guardian angel can fly ...
but keeping both feet on the ground wont get you anywhere :w00t:

- How to post Performance Problems
- How to post data/code to get the best help[/url]

- How to prevent a sore throat after hours of presenting ppt

press F1 for solution, press shift+F1 for urgent solution 😀

Need a bit of Powershell? How about this

Who am I ? Sometimes this is me but most of the time this is me

Elliott Whitlow SSC Guru Points: 102402 More actions · Answer 2

The answer is yes, sort of.. You can build a UDF that references another table in the same database, and use that UDF in the formula, I believe you can persist it.

If you can persist it DO! You don't want it hitting that UDF for every access to that column..

That UDF becomes bound to the table and cannot be changed without a mod to the table first to remove the reference.

so get it right the first time..

CEWII

Johan Bijnens SSC Guru Points: 135369 More actions · Answer 3

great... now you've had two answers saying No and Yes ...

That's what makes these fora good, you'll get alternatives and test hints.

I don't have time right now, but go on and just test the alternatives.

Pleas post feedback, so others can use your refs for their tests:cool:

In both cases you'll have to take into account there is a hidden join for your every application doing stuff with your table, unless this column is persisted.

Johan

Learn to play, play to learn !

Dont drive faster than your guardian angel can fly ...
but keeping both feet on the ground wont get you anywhere :w00t:

- How to post Performance Problems
- How to post data/code to get the best help[/url]

- How to prevent a sore throat after hours of presenting ppt

press F1 for solution, press shift+F1 for urgent solution 😀

Need a bit of Powershell? How about this

Who am I ? Sometimes this is me but most of the time this is me

Sean Lange SSC Guru Points: 286590 More actions · Answer 4

This was actually the topic of yesterday's QOTD.

Here is a link to the ensuing discussion.

_______________________________________________________________

Need help? Help us help you.

Read the article at http://www.sqlservercentral.com/articles/Best+Practices/61537/ for best practices on asking questions.

Need to split a string? Try Jeff Modens splitter http://www.sqlservercentral.com/articles/Tally+Table/72993/.

Cross Tabs and Pivots, Part 1 – Converting Rows to Columns - http://www.sqlservercentral.com/articles/T-SQL/63681/
Cross Tabs and Pivots, Part 2 - Dynamic Cross Tabs - http://www.sqlservercentral.com/articles/Crosstab/65048/
Understanding and Using APPLY (Part 1) - http://www.sqlservercentral.com/articles/APPLY/69953/
Understanding and Using APPLY (Part 2) - http://www.sqlservercentral.com/articles/APPLY/69954/

Johan Bijnens SSC Guru Points: 135369 More actions · Answer 5

Thanks for this addition Sean :Wow:

OP: Keep in mind it is not because you can do something in a certain way, you should. Think about the consequences.

Johan

Learn to play, play to learn !

Dont drive faster than your guardian angel can fly ...
but keeping both feet on the ground wont get you anywhere :w00t:

- How to post Performance Problems
- How to post data/code to get the best help[/url]

- How to prevent a sore throat after hours of presenting ppt

press F1 for solution, press shift+F1 for urgent solution 😀

Need a bit of Powershell? How about this

Who am I ? Sometimes this is me but most of the time this is me

Paul White SSC Guru Points: 150467 More actions · Answer 6

ningaraju.n (11/10/2010)
Is there a way to reference columns of other table in computed columns except triggers?

You could use a UDF that references the other table, but please don't. Functions that perform data access cannot be persisted, so it would be re-evaluated (effectively by running a separate query) once for every row it touches, every time. Absolutely horrible.

The idea of computed columns referencing other tables sounds like a VIEW to me.

Perhaps you could explain the circumstances more and provide an example to demonstrate what you are trying to achieve.

Again, please consider every possible alternative to using a function that does data access in a computed column.

Paul

Paul White
SQLPerformance.com
SQLkiwi blog
@SQL_Kiwi

Paul White SSC Guru Points: 150467 More actions · Answer 7

Code to demonstrate the issue, and show that UDFs that access data cannot be persisted:

CREATE TABLE dbo.Data

(

item INTEGER NOT NULL PRIMARY KEY,

value INTEGER NOT NULL,

);

INSERT dbo.Data (item, value)

SELECT V.number,

RAND(CHECKSUM(NEWID())) * 1000000

FROM master.dbo.spt_values V

WHERE V.type = N'P ';

GO

CREATE FUNCTION dbo.BadFunction(@item INTEGER)

RETURNS INTEGER

WITH SCHEMABINDING

AS

BEGIN

RETURN

(

SELECT D.value

FROM dbo.Data D

WHERE D.item = @item

)

END;

GO

-- Error if PERSITED keyword is uncommented:

-- Msg 4934, Level 16, State 3, Line 1

-- Computed column 'value' in table 'BadIdea' cannot be persisted

-- because the column does user or system data access.

CREATE TABLE dbo.BadIdea

(

row_id INTEGER PRIMARY KEY,

value AS

dbo.BadFunction (row_id)

--PERSISTED

);

GO

INSERT dbo.BadIdea(row_id)

SELECT V.number

FROM master.dbo.spt_values V

WHERE V.type = N'P ';

GO

-- Trace in Profiler to see separate function call

-- and query execution per row. (Does not show in

-- SSMS actual query plan).

-- Trace SQL:Batch Starting and SP:Starting

-- Shows 2048 separate calls to the UDF BadFunction.

-- Notice how long this simple query takes to run

-- on only 2048 rows.

SELECT MIN(value)

FROM dbo.BadIdea;

GO

DROP TABLE dbo.BadIdea;

DROP FUNCTION dbo.BadFunction;

DROP TABLE dbo.Data;

Paul White
SQLPerformance.com
SQLkiwi blog
@SQL_Kiwi

Johan Bijnens SSC Guru Points: 135369 More actions · Answer 8

Paul White NZ (11/15/2010)

If this message still is not understood, there will be no other means except for the server to blow up :w00t:

Thanks Paul for this clarification.

I still suffer SPMOS (SQLP*** Memory Overload Symptoms )

I'll need another couple of days until common sense returns. 😉

Johan

Learn to play, play to learn !

Dont drive faster than your guardian angel can fly ...
but keeping both feet on the ground wont get you anywhere :w00t:

- How to post Performance Problems
- How to post data/code to get the best help[/url]

- How to prevent a sore throat after hours of presenting ppt

press F1 for solution, press shift+F1 for urgent solution 😀

Need a bit of Powershell? How about this

Who am I ? Sometimes this is me but most of the time this is me