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Why is SQL Server index update deadlocked?
As the question notes, the general deadlock risk arises because the queries may obtain incompatible locks on the same resources in different orders. The SELECT query needs to access the index before the table due to the RID lookup, whereas the UPDATE query modifies the table first, then the index.
Is there a better way to avoid update statement locking out the table?
Does NOLOCK even make sense in this case, because the SQL Server would have to anyways get update locks for UPDATE. Is there a better way of achieving this? I know hints are to be avoided and SQL Server usually makes smarter choices, but I don’t want to get the table locked out during this update.
What causes a query to exceed the Lock escalation threshold?
This means that what may look like a SELECT statement at a “read-committed” isolation level might acquire many thousands of key locks (on both the clustered index and one nonclustered index). This can cause such a query to exceed the lock escalation thresholds.
What does it mean when index gets blocked in SQL?
So what that means is that SQL is scanning the clustered index from top to bottom until it hits the locked row. It can’t go any further at that point so ends up getting blocked.
How to get rid of deadlock in SQL Server?
The deadlock issue seems to be solved if clusted index or primary key is applied to the id field. I have different transactions doing one or more updates to different rows, e.g. transaction A will only update row with ID=a, tx B will only touch row with ID=b etc.
Why is my new index not getting blocked?
WOW, no deadlock! So what’s happening now. Let’s think about the update first, now be aware that because we’ve added an index, our update as also got to update that index too. Because of that, we’ll now see a lock on the new index as well. But why is our SELECT not getting blocked? Let’s have a look at that execution plan now…
How is indexing used to prevent deadlocks?
So by adding a covering index we can avoid our session getting blocked and prevent the deadlock from occurring. Just to prove that the scan is the cause of the block, we can add FORCESCAN to our query and see what happens. So now we can see that we’re once again scanning the index and now we’re back to the blocking situation.
How does a concurrent update cause a deadlock?
The reading side of the plan accesses the nonclustered index first, then the base table. The update side of the plan accesses the base table first, then the nonclustered index. By touching the same resources in reverse order, multiple concurrent executions of the same plan can deadlock.
When to use exclusive locks in SQL Server?
With a clustered index and a simple single-value equality predicate update, the query processor can apply an optimization that performs the update (read and write) in a single operator, using a single path: The row is located and updated in a single seek operation, requiring only exclusive locks (no update locks are needed).
What causes a deadlock on a bookmark lookup?
The deadlock graph shows that this particular deadlock was a conversion deadlock associated with a bookmark lookup (an RID lookup in this case): As the question notes, the general deadlock risk arises because the queries may obtain incompatible locks on the same resources in different orders.
Why does updlock, holdlock only apply to one table?
It seems odd to use a TABLE-level hint since this is an internal indexing issue – there is only one table involved – will UPDLOCK, HOLDLOCK automatically just apply to all indexes needed on that table and thereby force it to be serialized?
How to retry select if database is locked?
Setup retry logic within your application to automatically retry the select if it fails because of deadlock (or timeout), after waiting / sleeping for a brief period of time to allow the blocking queries to complete.