Changes to data during a sproc run

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Changes to data during a sproc run

bill36email

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Points: 37
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September 16, 2009 at 2:08 am

#97999

Hi,
I'm sure this has been answered but I'm not sure what to search for
If I have a stored procedure such as:
...
SELECT id
FROM sometable
WHERE columnA = 1
-- returns 10 rows
UPDATE sometable
SET someothercolumn = someothervalue
WHERE columnA = 1
-- updates 10 rows
...
What happens if the data in "sometable" is changed by another sproc between these 2 statements (e.g. an extra row is added where columnA=1). Is the set of rows changed by the UPDATE statement the same as the set of rows SELECTED by the first statement or is one extra row updated?
How can I ensure the same set of rows is addressed by each statement?
Thanks for any help! I just need to answer this definitively since I'm a bit lost with the transactionality options.

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Dave Ballantyne SSC-Dedicated Points: 33667 More actions · Answer 1

It depends on your transaction isolation level,

http://msdn.microsoft.com/en-us/library/ms173763.aspx.

The question is what do you want to happen ?

Do you want the other process blocked ?

Do you want the update to see the new data ?

Clear Sky SQL
My Blog[/url]

bill36email SSC Rookie Points: 37 More actions · Answer 2

Thanks Dave. It seems I'd need to add

SET TRANSACTION ISOLATION LEVEL SNAPSHOT

since I'd like to disregard any changes made outside the current process. It appears that I can set this isolation level for the sproc and it will return to the default isolation level once the sproc has completed, is that correct?

Dave Ballantyne SSC-Dedicated Points: 33667 More actions · Answer 3

You could do , i believe that SNAPSHOT has a fairly high overhead.

Arguably this could be better

SELECT id

into #IdsToupdate

FROM sometable

WHERE columnA = 1

UPDATE sometable

SET someothercolumn = someothervalue

from #IdsToupdate

WHERE #IdsToupdate.Id = sometable.Id

Clear Sky SQL
My Blog[/url]

bill36email SSC Rookie Points: 37 More actions · Answer 4

That's exactly what I was doing but I thought there had to be another way. Do you think that the Snapshot solution would incur too much of an overhead? The sproc is run once every 10 minutes and doesn't operate on a vast amount of data.

Thanks for the help!

Dave Ballantyne SSC-Dedicated Points: 33667 More actions · Answer 5

As ever , it depends. Personally ive never used snapshot isolation , so cant comment further really.

Try this link though

http://www.sqlnewsgroups.net/group/microsoft.public.sqlserver.server/topic11164.aspx

As to wheater its acceptable in your environment with your data, its all down to testing.

Clear Sky SQL
My Blog[/url]

Jeff Moden SSC Guru Points: 1004600 More actions · Answer 6

bill36email (9/16/2009)
That's exactly what I was doing but I thought there had to be another way. Do you think that the Snapshot solution would incur too much of an overhead? The sproc is run once every 10 minutes and doesn't operate on a vast amount of data.
Thanks for the help!

There is another way... lookup the OUTPUT clause in Books Online and forget about using the SELECT statement altogether. Lot's of examples on that thread, as well.

--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)

Jeff Moden SSC Guru Points: 1004600 More actions · Answer 7

I still recommend you lookup and study OUTPUT in Books Online, but here's a working example...

--===== Create and populate a 1,000,000 row test table.

-- Column "RowNum" has a range of 1 to 100,000 unique numbers

-- Column "SomeInt" has a range of 1 to 50,000 non-unique numbers

-- Column "SomeLetters2" has a range of "AA" to "ZZ" non-unique 2 character strings

-- Column "SomeMoney has a range of 0.0000 to 99.9999 non-unique numbers

-- Column "SomeDate" has a range of >=01/01/2000 and <01/01/2010 non-unique date/times

-- Column "SomeCSV" contains 'Part01,Part02,Part03,Part04,Part05,Part06,Part07,Part08,Part09,Part10'

-- for all rows.

-- Column "SomeHex12" contains 12 random hex characters (ie, 0-9,A-F)

-- Jeff Moden

SELECT TOP 1000000

SomeID = IDENTITY(INT,1,1),

SomeInt = ABS(CHECKSUM(NEWID()))%50000+1,

SomeLetters2 = CHAR(ABS(CHECKSUM(NEWID()))%26+65)

+ CHAR(ABS(CHECKSUM(NEWID()))%26+65),

SomeCSV = CAST('Part01,Part02,Part03,Part04,Part05,Part06,Part07,Part08,Part09,Part10' AS VARCHAR(80)),

SomeMoney = CAST(ABS(CHECKSUM(NEWID()))%10000 /100.0 AS MONEY),

SomeDate = CAST(RAND(CHECKSUM(NEWID()))*3653.0+36524.0 AS DATETIME),

SomeHex12 = RIGHT(NEWID(),12)

INTO dbo.JBMTest

FROM Master.dbo.SysColumns t1,

Master.dbo.SysColumns t2 --Lack of join criteria makes this a CROSS-JOIN

--===== A table is not properly formed unless a Primary Key has been assigned

-- Takes about 1 second to execute.

ALTER TABLE dbo.JBMTest

ADD PRIMARY KEY CLUSTERED (SomeID)

--===== Do the combination of the SELECT and UPDATE using just an UPDATE

UPDATE dbo.JBMTest

SET SomeCSV = 'Changed'

OUTPUT Deleted.SomeID,

Deleted.SomeCSV AS BeforeChange,

Inserted.SomeCSV AS AfterChange

WHERE SomeInt = 1000

--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)

Changes to data during a sproc run

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