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RE: Creating indexes on computed columns

Konstantin Reu — Thu, 05 Jan 2012 00:59:59 GMT

There is error. Correcty answer is 1 and 2. But 4 "Has no system data access or user data access." is correct when computed column marks as persisted and based on function, that will not have access to user/system tables.Does question "Creating indexes on computed columns" contain any about persisted ? - No.... I don't won 1 point.

RE: Creating indexes on computed columns

L' Eomot Inversé — Fri, 01 Oct 2010 14:05:47 GMT

Two interesting things here: The first is that the BoL page referenced in the explanation has incorrect information on it: it quite clearly says [quote]A computed_column_expression is deterministic when one or more of the following is true:[/quote] where actually all need to true, not just one.The second is that the corrected answer may still be wrong: as someone pointed out in an earlier comment, the BoL page says (twice) that a computed column which is imprecise can be used in an index provided it is persisted. So perhaps item 1 should not be in the list of answers to be checked.

RE: Creating indexes on computed columns

SQLRNNR — Thu, 30 Sep 2010 18:40:45 GMT

[quote][b]Steve Jones - Editor (9/30/2010)[/b][hr]The answer choices have been corrected and points awarded back.[/quote]Thanks for fixing that - for those of us answering late.

RE: Creating indexes on computed columns

gchornenkyy — Thu, 30 Sep 2010 12:17:17 GMT

Just wonder about user data meaning. If I have user defined table it means that it keeps user defined data.Here the very simple example of filtered index on computed columnCREATE TABLE [dbo].[t2]( [c2] [int] NULL, [comp_col] AS ([c2]*(2)) PERSISTED) ON [PRIMARY]GOcreate index ix_comp_col on t2 ( comp_col )where c2 > 4GOSo?

RE: Creating indexes on computed columns

Steve Jones - SSC Editor — Thu, 30 Sep 2010 09:22:38 GMT

The answer choices have been corrected and points awarded back.

RE: Creating indexes on computed columns

hkamal.infoedge — Thu, 30 Sep 2010 08:45:07 GMT

No column reference pulls data from multiple rows. For example, aggregate functions such as SUM or AVG depend on data from multiple rows and would make a computed_column_expression nondeterministic.

RE: Creating indexes on computed columns

gajapathy — Thu, 30 Sep 2010 07:55:53 GMT

Hi,As others did, I have selected choices 1,2 and 4 and left 3 unchecked. If this is wrong then MSDN itself wrong. Please refer the linkhttp://msdn.microsoft.com/en-us/library/ms189292.aspxI am not responding to gain the point, but to get exactly which is correct answer.Regards,

RE: Creating indexes on computed columns

Christian Buettner-167247 — Thu, 30 Sep 2010 07:50:12 GMT

Hi Steve,[quote][b]Steve Eckhart (9/30/2010)[/b][hr]Actually, if you read carefully, an index on a computed column CAN include multiple rows and columns from other tables. [/quote]Can you give an example of multiple rows? I doubt it.And for the columns from other tables -> That is per definition non-deterministic.[url]http://msdn.microsoft.com/en-us/library/aa214775(SQL.80).aspx[/url][quote]As long as [i]one[/i] of the following four conditions are true, then you can use the computed column in an index.[/quote]Now that does not say "exactly" one, it means one or more. It does not tell you which combinations would be deterministic and precise.

RE: Creating indexes on computed columns

daveb87 — Thu, 30 Sep 2010 07:33:13 GMT

Please check the answers before posting a question. People use the QofD as a way to learn or to refresh the memory on a given topic. Bad and incorrect questions just waste time.

RE: Creating indexes on computed columns

gary.mazzone — Thu, 30 Sep 2010 07:31:09 GMT

I don't mind getting a question wrong, but only when the answer i give is the wrong one. The third choice is clearly incorrect as true when read in BOL.

RE: Creating indexes on computed columns

Steve Eckhart — Thu, 30 Sep 2010 07:13:51 GMT

Actually, if you read carefully, an index on a computed column CAN include multiple rows and columns from other tables. As long as [i]one[/i] of the following four conditions are true, then you can use the computed column in an index. Therefore, only the first answer is necessary and the other three are incorrect answers.[quote]The computed_column_expression must be deterministic. A computed_column_expression is deterministic when one or more of the following is true: All functions that are referenced by the expression are deterministic and precise. These functions include both user-defined and built-in functions. For more information, see Deterministic and Nondeterministic Functions. Functions might be imprecise if the computed column is PERSISTED. For more information, see Creating Indexes on Persisted Computed Columns later in this topic.All columns that are referenced in the expression come from the table that contains the computed column.No column reference pulls data from multiple rows. For example, aggregate functions such as SUM or AVG depend on data from multiple rows and would make a computed_column_expression nondeterministic.Has no system data access or user data access.[/quote]

RE: Creating indexes on computed columns

JestersGrind — Thu, 30 Sep 2010 07:12:21 GMT

Very disappointing. This is a fairly basic question. A little proofreading could have prevented this from becoming a bad question.

RE: Creating indexes on computed columns

sjimmo — Thu, 30 Sep 2010 06:45:58 GMT

Manie: Your answers are wrong. You state that you [b]can[/b] pull data from multiple rows, yet BOL which you state as your reference says [quote]No column reference pulls data from multiple rows. For example, aggregate functions such as SUM or AVG depend on data from multiple rows and would make a computed_column_expression nondeterministic.[/quote]Can't have it both ways.

RE: Creating indexes on computed columns

Toreador — Thu, 30 Sep 2010 06:29:20 GMT

"Me too" on the incorrect options stuff.But in any case, 1 point is a bit mean for a 4-checkbox multiselect question

RE: Creating indexes on computed columns

rtelgenhoff — Thu, 30 Sep 2010 05:58:41 GMT

Yes,http://msdn.microsoft.com/en-us/library/ms189292.aspx#BKMK_persisted states that:"You can create an index on a computed column that is defined with a deterministic, but imprecise, expression if the column is marked PERSISTED in the CREATE TABLE or ALTER TABLE statement."

RE: Creating indexes on computed columns

William Vach — Thu, 30 Sep 2010 05:37:04 GMT

The explanation contradicts answer number 3 being true.

RE: Creating indexes on computed columns

george sibbald — Thu, 30 Sep 2010 03:35:28 GMT

please see [url]http://msdn.microsoft.com/en-us/library/ms189292.aspx[/url], the third answer is incorrect, I am sure Steve will rectify the situation.this actually follows on from the question two days ago, which is why I answered it.

RE: Creating indexes on computed columns

kaspencer — Thu, 30 Sep 2010 03:25:23 GMT

Yet another error. I won't tolerate any mealy-mouthed excuses, such as those which we have experienced in the past. Go to the very bottom of the class, and stay there with your dunces cap on, until I permit you to move.[And give me my point]Kenneth Spencer.

RE: Creating indexes on computed columns

Imran Ashraf-452633 — Thu, 30 Sep 2010 02:55:57 GMT

Like the many others I have select the 3 options and not selected Column references can pull data from multiple rows. From what I can tell I was right like the others so give me my 1 point:angry:

RE: Creating indexes on computed columns

paul.knibbs — Thu, 30 Sep 2010 02:46:22 GMT

I carefully left that third option unchecked because I thought it was the author attempting a bit of a trick by having just that one worded differently from BOL, too!

RE: Creating indexes on computed columns

michael.kaufmann — Thu, 30 Sep 2010 02:22:56 GMT

[quote][b]UMG Developer (9/29/2010)[/b][hr]I think you need to check the answer key for this question. It lists "Column references can pull data from multiple rows." as one of the options you have to pick, but then in your explanation you say "No column reference pulls data from multiple rows."And the BOL article you link to says 'No column reference pulls data from multiple rows.", so it seems that "Column references can pull data from multiple rows." is not a correct choice. Either that or you meant for the [b]can[/b] to be [b]can't[/b].[/quote]Fully agree with UMG Developer.And I'd also agree with Chris Büttner--but in order to gain a point, just stick with the wording in BOL and disregard anything else, as computed columns need to be [b]persisted[/b] in order to be used in an index (and the question was only about 'computed columns' but not about 'persisted computed columns'). :cool:Regards,Michael

RE: Creating indexes on computed columns

Christian Buettner-167247 — Thu, 30 Sep 2010 01:43:57 GMT

The first answer (All functions that are referenced by the expression are deterministic and precise.) is incorrect as well. As long as the column is persisted, imprecise functions can be used.

RE: Creating indexes on computed columns

Hardy21 — Thu, 30 Sep 2010 01:23:16 GMT

Though I am right, I lose point :(

RE: Creating indexes on computed columns

Carlo Romagnano — Thu, 30 Sep 2010 01:09:30 GMT

Here the script that states "No column reference pulls data from multiple rows"create function aaaa() returns intasbegin return (select SUM(a) from aza )endGOcreate table aza(a int,b as dbo.aaaa())create index idx_aza on aza(b) -- raise error: function is NON-DETERMINISTIC

RE: Creating indexes on computed columns

honza.mf — Thu, 30 Sep 2010 01:04:10 GMT

Just another 0=1

RE: Creating indexes on computed columns

Kingston Dhasian — Wed, 29 Sep 2010 23:00:29 GMT

[quote][b]UMG Developer (9/29/2010)[/b][hr]I think you need to check the answer key for this question. It lists "Column references can pull data from multiple rows." as one of the options you have to pick, but then in your explanation you say "No column reference pulls data from multiple rows."And the BOL article you link to says 'No column reference pulls data from multiple rows.", so it seems that "Column references can pull data from multiple rows." is not a correct choice. Either that or you meant for the [b]can[/b] to be [b]can't[/b].[/quote]I agree. The answer key for the question is wrong. The explanation is correct as per BOL. I think the editors will rectify this soon.

RE: Creating indexes on computed columns

Oleg Netchaev — Wed, 29 Sep 2010 22:01:34 GMT

Wow, this is so ridiculous. The referenced BOL page clearly states:[quote]No column reference pulls data from multiple rows[/quote]The answer, however includes the opposite and still claims it as a correct option. As a matter of fact even the wording of all listed options matches the BOL word by word except it does not include the word [b]NO[/b] for the multiple rows option.Oleg

RE: Creating indexes on computed columns

UMG Developer — Wed, 29 Sep 2010 21:52:19 GMT

I think you need to check the answer key for this question. It lists "Column references can pull data from multiple rows." as one of the options you have to pick, but then in your explanation you say "No column reference pulls data from multiple rows."And the BOL article you link to says 'No column reference pulls data from multiple rows.", so it seems that "Column references can pull data from multiple rows." is not a correct choice. Either that or you meant for the [b]can[/b] to be [b]can't[/b].

Creating indexes on computed columns

Manie Verster — Wed, 29 Sep 2010 21:49:10 GMT

Comments posted to this topic are about the item [B]Creating indexes on computed columns[/B]