Best way to batchprocess a large update
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When you use the
TOPclause with theUPDATEstatement, there's no guarantee that the rows to update will be picked in any particular order. Using theMAX(id)option, you could end up missing rows. I notice you've replaced the temporary table with a table variable. Was there a reason for that? IIRC, execution plans for table variables tend to assume they contain a very low number of rows, which might explain the poor performance.
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
Richard Deeming wrote:
When you use the
TOPclause with theUPDATEstatement, there's no guarantee that the rows to update will be picked in any particular order. Using theMAX(id)option, you could end up missing rows.I know, and you can't add an order by to an UPDATE or INSERT. But you can put the SELECT with TOP and ORDER BY in a CTE.
Richard Deeming wrote:
I notice you've replaced the temporary table with a table variable. Was there a reason for that?
No particular reason. I like to keep the scope as local as possible, so it's mostly a habit.
Richard Deeming wrote:
IIRC, execution plans for table variables tend to assume they contain a very low number of rows, which might explain the poor performance.
Table variables don't have statistics, which obviously could affect the plan, but since all ID's are unique I don't think it would make a big difference in this case But I will test it. <edit>Oh, and table variables can't go parallell, which obviously can affect performance a lot in this case.</edit>
Wrong is evil and must be defeated. - Jeff Ello
-
When you use the
TOPclause with theUPDATEstatement, there's no guarantee that the rows to update will be picked in any particular order. Using theMAX(id)option, you could end up missing rows. I notice you've replaced the temporary table with a table variable. Was there a reason for that? IIRC, execution plans for table variables tend to assume they contain a very low number of rows, which might explain the poor performance.
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
Done some testing now. And as I suspected, there is no difference in either performance or plan as long as the temp table has one column with unique values. Until the query goes parallel that is. Then the difference is quickly getting huge. But as long as I'm batching the query it stays the same until the batch is big enough to go parallel (which happens between 10000 and 20000 rows in this case). But then I will also get a table lock. And oddly enough, it also goes much slower when parallel until reaching 100000 rows per batch. I will do some more testing on this.
Wrong is evil and must be defeated. - Jeff Ello
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So I need to regularly update a table with data from another table. The problem is that if I update the normal way I get a table lock on the target table for half an hour, which is frowned upon by the users. So I need to run the update in batches. The other problem is that the ID sequence is having gaps in it. Larger gaps than the batch size. At the moment I have this solution:
DECLARE
@LastID int = 0,
@NextID int,
@RC int = 1;WHILE (@RC > 0)
BEGIN
SELECT TOP 5000
@NextID = s.id
FROM Source s
WHERE s.id> @LastID
ORDER BY s.id
;
UPDATE t
SET ------
FROM Source s
JOIN Target t ON t.id = s.id
WHERE s.id > @LastID
AND s.id <= @NextID
;
SET @RC = @@ROWCOUNT;
SET @LastID = @NextID ;
ENDWhich works just fine, but using two selects is getting under my skin. Any better suggestions for how to do it?
Wrong is evil and must be defeated. - Jeff Ello
Add WITH(NOLOCK) to your selects and joins:
DECLARE
@LastID int = 0,
@NextID int,
@RC int = 1;WHILE (@RC > 0)
BEGIN
SELECT TOP 5000
@NextID = s.id
FROM Source s WITH(NOLOCK)
WHERE s.id> @LastID
ORDER BY s.id
;
UPDATE t
SET ------
FROM Source s
JOIN Target t WITH(NOLOCK) ON t.id = s.id
WHERE s.id > @LastID
AND s.id <= @NextID
;
SET @RC = @@ROWCOUNT;
SET @LastID = @NextID ;
END".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
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When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013 -
Add WITH(NOLOCK) to your selects and joins:
DECLARE
@LastID int = 0,
@NextID int,
@RC int = 1;WHILE (@RC > 0)
BEGIN
SELECT TOP 5000
@NextID = s.id
FROM Source s WITH(NOLOCK)
WHERE s.id> @LastID
ORDER BY s.id
;
UPDATE t
SET ------
FROM Source s
JOIN Target t WITH(NOLOCK) ON t.id = s.id
WHERE s.id > @LastID
AND s.id <= @NextID
;
SET @RC = @@ROWCOUNT;
SET @LastID = @NextID ;
END".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013But only if you understand the risks first. :) Using NOLOCK? Here's How You'll Get the Wrong Query Results. - Brent Ozar Unlimited®[^] Bad habits : Putting NOLOCK everywhere - SQL Sentry[^] Also, it won't work on the target table: Avoid using NOLOCK on SQL Server UPDATE and DELETE statements[^]
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
"These people looked deep within my soul and assigned me a number based on the order in which I joined" - Homer
-
But only if you understand the risks first. :) Using NOLOCK? Here's How You'll Get the Wrong Query Results. - Brent Ozar Unlimited®[^] Bad habits : Putting NOLOCK everywhere - SQL Sentry[^] Also, it won't work on the target table: Avoid using NOLOCK on SQL Server UPDATE and DELETE statements[^]
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
I didn't put nolock on the update statement - I put it on the join. You could just create a job that does the monster update at night.
".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013 -
I didn't put nolock on the update statement - I put it on the join. You could just create a job that does the monster update at night.
".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013Quote:
UPDATE t
...
FROM Source s
JOIN Target t WITH(NOLOCK) ON t.id = s.id
...That
NOLOCKhint is on the target table. It's exactly the same as the first example from the article I linked to:Avoid using NOLOCK on SQL Server UPDATE and DELETE statements[^]:
UPDATE t1
SET t1.x = something
FROM dbo.t1 WITH (NOLOCK)
INNER JOIN ...;
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
-
So I need to regularly update a table with data from another table. The problem is that if I update the normal way I get a table lock on the target table for half an hour, which is frowned upon by the users. So I need to run the update in batches. The other problem is that the ID sequence is having gaps in it. Larger gaps than the batch size. At the moment I have this solution:
DECLARE
@LastID int = 0,
@NextID int,
@RC int = 1;WHILE (@RC > 0)
BEGIN
SELECT TOP 5000
@NextID = s.id
FROM Source s
WHERE s.id> @LastID
ORDER BY s.id
;
UPDATE t
SET ------
FROM Source s
JOIN Target t ON t.id = s.id
WHERE s.id > @LastID
AND s.id <= @NextID
;
SET @RC = @@ROWCOUNT;
SET @LastID = @NextID ;
ENDWhich works just fine, but using two selects is getting under my skin. Any better suggestions for how to do it?
Wrong is evil and must be defeated. - Jeff Ello
Have you tried using a
MERGEstatement?".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013 -
Quote:
UPDATE t
...
FROM Source s
JOIN Target t WITH(NOLOCK) ON t.id = s.id
...That
NOLOCKhint is on the target table. It's exactly the same as the first example from the article I linked to:Avoid using NOLOCK on SQL Server UPDATE and DELETE statements[^]:
UPDATE t1
SET t1.x = something
FROM dbo.t1 WITH (NOLOCK)
INNER JOIN ...;
"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer
We use WITH(NOLOCK) prolifically. Of course, we have indexes on all of our tables, and don't generally do massive updates in the middle of the work day. We have no issues.
".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013 -
We use WITH(NOLOCK) prolifically. Of course, we have indexes on all of our tables, and don't generally do massive updates in the middle of the work day. We have no issues.
".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013NOLOCK can cause nonclustered index corruption, and it's also deprecated[^].
Wrong is evil and must be defeated. - Jeff Ello
-
Have you tried using a
MERGEstatement?".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013Yes. The update in my OP is greatly simplified. The actual update is a merge with more than 4000 rows.
Wrong is evil and must be defeated. - Jeff Ello
-
NOLOCK can cause nonclustered index corruption, and it's also deprecated[^].
Wrong is evil and must be defeated. - Jeff Ello
That's something for our DBAs to worry about. :)
".45 ACP - because shooting twice is just silly" - JSOP, 2010
-----
You can never have too much ammo - unless you're swimming, or on fire. - JSOP, 2010
-----
When you pry the gun from my cold dead hands, be careful - the barrel will be very hot. - JSOP, 2013
