Computed Columns

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jghali

Posted Friday, July 23, 2010 7:33 AM

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Very well said Hugo.

That's pretty much what I was trying to say although I wasn't thinking of Space (since today, disk space is pretty cheap)... your explanation is excellent.

Thank you for clarifying.
JGhali

Post #957954

hrvoje.piasevoli

Posted Friday, July 23, 2010 11:40 AM

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I guess now would be a good time to point to a great post by PaulWhiteNz about UDFs and SCHEMABINDING which I can not find right now. My read performance is quite good while write sucks as I am on my iPhone;)
Paul could you quote yourself please?

Here is the link to a discussion on SSC: http://www.sqlservercentral.com/Forums/Topic737008-360-1.aspx#bm738974

Hrvoje Piasevoli

Post #958141

Paul White

Posted Friday, July 23, 2010 5:50 PM

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hrvoje.piasevoli (7/23/2010)

Will this do?
http://blogs.msdn.com/b/sqlprogrammability/archive/2006/05/12/596424.aspx

Paul White
SQL Server MVP
SQLblog.com
@SQL_Kiwi

Post #958336

Toreador

Posted Monday, July 26, 2010 3:06 AM

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I hate "me too" replies, but I also failed on the "Will definitely use more resources" option, because it is just not true. Consider the alternative of a standard column populated by a trigger. A trigger will update the column on every insert and every relevant update; whereas the computed column will only be evaluated when it is selected. If you never include the column in a select statement then how can it use more resources? And yes I know that you wouldn't create a computed column that you don't intend to use, but a seldom-accessed computed column is still going to use fewer resources than a trigger.

I didn't choose "The engine tweaks to be performant." as I have no idea what it means!

Final whinge, it's a shame that these multiple-check-box question give the marks on an all-or-nothing basis; choosing 4 out of 5 correct options scores the same as choosing no correct options but every incorrect one BigGrin

Post #958651

hakan.winther

Posted Monday, August 16, 2010 6:12 AM

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Nice question, and as everyone else pointed out, the computed columns doesn't need to user more resources. Hugo gave an excellent example on how it could use less resources.

/Håkan Winther
MCITP:Database Developer 2008

Post #969708

rtelgenhoff

Posted Saturday, October 02, 2010 4:57 PM

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In the example,

create table a
(
col1 int,
col2 int,
col3 as col1,
)

col3 is technically a computed column, but uses only an extremely low amount of cpu time, especially since it does only a simple in-memory copy operation, but since it's not read from a disk buffer, is probably actually more efficient than a normal column. The statement 'definitely uses more resources' may be a stretch. I would agree that an expression utilizing a SELECT or UDF would use more resources and could potentially cause performance issues.

Post #997251

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