Order of rows in a Table with Identity Column.

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Order of rows in a Table with Identity Column.

jignesh209

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July 1, 2013 at 7:34 pm

#279792

I have a table with an Identity Column as TransactionId and also it is a primary key.
The table has around 9 Million rows.
The table also has number of columns and in that one is OrderNumer.
Now,if I query it by SELECT * FROM OrderTransaction where OrderNumber=89856.It returns 2 rows.
TransactionId OrderNumber
1015 89856
1016 89856
Will the query(this query with no ORDER BY) always return in the order mentioned above or it will return rows in the order as:
TransactionId OrderNumber
1016 89856
1015 89856

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Lynn Pettis SSC Guru Points: 442467 More actions · Answer 1

jerry209 (7/1/2013)
I have a table with an Identity Column as TransactionId and also it is a primary key.
The table has around 9 Million rows.
The table also has number of columns and in that one is OrderNumer.
Now,if I query it by SELECT * FROM OrderTransaction where OrderNumber=89856.It returns 2 rows.
TransactionId OrderNumber
1015 89856
1016 89856
Will the query(this query with no ORDER BY) always return in the order mentioned above or it will return rows in the order as:
TransactionId OrderNumber
1016 89856
1015 89856

The only way to guarantee order you must use an ORDER BY clause in your query. SQL Server will not guarantee the order of the data otherwise.

Joie Andrew One Orange Chip Points: 27360 More actions · Answer 2

The only way to guarantee order you must use an ORDER BY clause in your query. SQL Server will not guarantee the order of the data otherwise.

+1

Set theory does not guarantee the order of a result set. The only way to guarantee it is with an order by clause (or have your app sort the results).

Joie Andrew
"Since 1982"

Sean Lange SSC Guru Points: 286590 More actions · Answer 3

Check out this article that explains what is happening and some great example to help you understand ordering.

http://blogs.msdn.com/b/conor_cunningham_msft/archive/2008/08/27/no-seatbelt-expecting-order-without-order-by.aspx

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Dwain Camps SSC Guru Points: 86948 More actions · Answer 4

Excuse me for asking a dumb question here but the OP says the TransactionID is the primary key with presumably a clustered index on it (my assumption).

In that case, without an ORDER BY shouldn't the rows be returned in the order established by the clustered INDEX?

Otherwise, how would a QU ever work?

My mantra: No loops! No CURSORs! No RBAR! Hoo-uh![/I]
My thought question: Have you ever been told that your query runs too fast?

My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.

Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]
[url url=http://www.sqlservercentral.com/articles/St

Joie Andrew One Orange Chip Points: 27360 More actions · Answer 5

I could be wrong, but I believe a clustered index is for how the data is stored, not retrieved. From what I've read the optimizer can take various ways of retrieving rows depending on what it thinks is fastest. If you have a table with pages split across several disks there could be multiple read operations bringing back different sets of data at different times. Without an order by clause it will return the data in whatever order it gets it in first; all it has to guarantee is the data, not the order of the data.

Joie Andrew
"Since 1982"

Jeff Moden SSC Guru Points: 1004309 More actions · Answer 6

dwain.c (7/8/2013)
Excuse me for asking a dumb question here but the OP says the TransactionID is the primary key with presumably a clustered index on it (my assumption).
In that case, without an ORDER BY shouldn't the rows be returned in the order established by the clustered INDEX?
Otherwise, how would a QU ever work?

That's a part of the bone of contention that some folks have with the Quirky Update. The QU depends on the use of the Clustered Index AND that it's used in an "ORDERED" fashion. That's the reason for all the rules such a MAXDOP 1 and referencing the lead column of the CI, etc. Paul White came up with a method (referenced at the beginning of the "latest" article) that uses a CTE with a RowNumber and a counter in the outer query that must match or it gives you an error. Oddly enough, the method seems to guarantee that if the QU is setup correctly to run to begin with, it'll never error.

As for relying on the CU for a "natural" sort of other things goes, it's not guaranteed especially if there's another unique index on the table. Other than some of the trickery in the rules of a QU, the only way to guarantee order in a SELECT is to use an ORDER BY. If SQL Server decides to use the CI in an "ORDERED" mode, it'll ignore the sort... it won't even include it in the execution plan.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 7

dwain.c (7/8/2013)
In that case, without an ORDER BY shouldn't the rows be returned in the order established by the clustered INDEX?

No. Without an order by there is no guarantee of the order that the rows will be returned in. Now, for a simple select * without parallelism, the data will probably come back in the order of the index used for the query, because that's the order the QP left it in after the last query operator. However don't confuse probable behaviour with guaranteed behaviour, without an order by there is no guaranteed order.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

Dwain Camps SSC Guru Points: 86948 More actions · Answer 8

Jeff and Gila,

Thanks for the clarifications. I take it you're saying that even with a MAXDOP 1 on the simple SELECT it doesn't guarantee row ordering.

My question was mostly speculative because I would always write it with an ORDER BY to be certain. I probably should have added a "usually" to my comment.

However I would be interested to see a case where a simple SELECT doesn't return rows ordered by the CI. I am not doubting either of you when I say this, just trying to learn more about it.

I also realize that doing other things in the SELECT (e.g., JOIN to another table) would certainly lower the likelihood of rows returned in the CI order.

Jeff - I am unclear on where to find the article you referenced from Paul White.

My mantra: No loops! No CURSORs! No RBAR! Hoo-uh![/I]
My thought question: Have you ever been told that your query runs too fast?

My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.

Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]
[url url=http://www.sqlservercentral.com/articles/St

Jeff Moden SSC Guru Points: 1004309 More actions · Answer 9

It wasn't an article. It was a post on the Running Total article and Tom updated it with a slightly different take on the check. Tom's post with both his an Paul White's code puts it altogether quite nicely at the following post, the URL for which is also posted at the beginning of the Running Total article. Heres' the link for Paul and Tom's contribution.

http://www.sqlservercentral.com/Forums/FindPost981258.aspx

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Dwain Camps SSC Guru Points: 86948 More actions · Answer 10

Thanks for the link Jeff. That is some pretty deep stuff.

My mantra: No loops! No CURSORs! No RBAR! Hoo-uh![/I]
My thought question: Have you ever been told that your query runs too fast?

My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.

Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]
[url url=http://www.sqlservercentral.com/articles/St

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 11

dwain.c (7/9/2013)
Thanks for the clarifications. I take it you're saying that even with a MAXDOP 1 on the simple SELECT it doesn't guarantee row ordering.

The only thing that guarantees an order is the order by. Everything else is just relying on current behaviour.

However I would be interested to see a case where a simple SELECT doesn't return rows ordered by the CI. I am not doubting either of you when I say this, just trying to learn more about it.

Parallelism, allocation-order scan, query satisfied using an index other than the clustered index, and that's just off the top of my head.

There's an example on Conor's blog http://blogs.msdn.com/b/conor_cunningham_msft/archive/2008/08/27/no-seatbelt-expecting-order-without-order-by.aspx?Redirected=true In his case, it's parallelism kicking in.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

Dwain Camps SSC Guru Points: 86948 More actions · Answer 12

GilaMonster (7/10/2013)
dwain.c (7/9/2013)
Thanks for the clarifications. I take it you're saying that even with a MAXDOP 1 on the simple SELECT it doesn't guarantee row ordering.
The only thing that guarantees an order is the order by. Everything else is just relying on current behaviour.
However I would be interested to see a case where a simple SELECT doesn't return rows ordered by the CI. I am not doubting either of you when I say this, just trying to learn more about it.
Parallelism, allocation-order scan, query satisfied using an index other than the clustered index, and that's just off the top of my head.
There's an example on Conor's blog http://blogs.msdn.com/b/conor_cunningham_msft/archive/2008/08/27/no-seatbelt-expecting-order-without-order-by.aspx?Redirected=true In his case, it's parallelism kicking in.

Thanks Gail! That article gave a very clear description.

My mantra: No loops! No CURSORs! No RBAR! Hoo-uh![/I]
My thought question: Have you ever been told that your query runs too fast?

My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.

Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]
[url url=http://www.sqlservercentral.com/articles/St