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READ_COMMITTED_SNAPSHOT & TRUNCATE TABLE


READ_COMMITTED_SNAPSHOT & TRUNCATE TABLE

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geoffrey crombez
geoffrey crombez
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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
Sean Lange
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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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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


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Sean Lange
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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. w00t

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