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SQL Server 2005
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SQL Server 2005 Strategies
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READ_COMMITTED_SNAPSHOT & TRUNCATE TABLE
READ_COMMITTED_SNAPSHOT & TRUNCATE TABLE
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geoffrey crombez
geoffrey crombez
Posted Friday, December 21, 2012 3:45 PM
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Last Login: Wednesday, April 17, 2013 1:40 AM
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Hi,
when I use the READ_COMMITTED_SNAPSHOT and use DELETE FROM table the other session is not locked when SELECTING data.
However when I use TRUNCATE TABLE it does block the other session SELECTING data.
The difference I see is that the TRUNCATE does a X lock on PAGE level and the DELETE an IX on PAGE level.
So I know not to use TRUNCATE in such scenario, but I would like to understand what is different internally.
Kind Regards
Geoffrey
Post #1399620
Sean Lange
Sean Lange
Posted Wednesday, December 26, 2012 7:31 AM
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geoffrey crombez (12/21/2012)
Hi,
when I use the READ_COMMITTED_SNAPSHOT and use DELETE FROM table the other session is not locked when SELECTING data.
However when I use TRUNCATE TABLE it does block the other session SELECTING data.
The difference I see is that the TRUNCATE does a X lock on PAGE level and the DELETE an IX on PAGE level.
So I know not to use TRUNCATE in such scenario, but I would like to understand what is different internally.
Kind Regards
Geoffrey
The difference in what those two statements do. Delete simply deletes rows where a truncate removes the pages. The locking you are seeing is because of lock escalation. A truncate will lock the entire table because all of the pages are being removed. Of course truncate is a lot faster because of the minimal logging involved so the locking should be very short.
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GilaMonster
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Posted Wednesday, December 26, 2012 1:04 PM
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Sean Lange (12/26/2012)
The locking you are seeing is because of lock escalation.
It's not lock escalation.
Truncate table locks at the table level because there's no other sensible level to lock at (and iirc it takes an Sch-M lock too)
Lock escalation would be if the truncate first started taking row or page locks, got over the threshold and converted the locks to table
Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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Post #1400349
Sean Lange
Sean Lange
Posted Wednesday, December 26, 2012 1:07 PM
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GilaMonster (12/26/2012)
Sean Lange (12/26/2012)
The locking you are seeing is because of lock escalation.
It's not lock escalation.
Truncate table locks at the table level because there's no other sensible level to lock at (and iirc it takes an Sch-M lock too)
Lock escalation would be if the truncate first started taking row or page locks, got over the threshold and converted the locks to table
Thanks for the correction Gail. I knew the locking was at the table level but my incorrect terminology certainly made it unclear.
_______________________________________________________________
Need help? Help us help you.
Read the article at
http://www.sqlservercentral.com/articles/Best+Practices/61537/
for best practices on asking questions.
Need to split a string? Try Jeff Moden's
splitter
.
Cross Tabs and Pivots, Part 1 – Converting Rows to Columns
Cross Tabs and Pivots, Part 2 - Dynamic Cross Tabs
Post #1400351
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