Question

UPDLOCK implications

Asked by: jasonstephens

To try to resolve deadlocks in my DB, as per recommendations in BOL and MSDN, I'm going through my transactions, and, in cases where they:

1. select rows from a deadlocking table
2. do lots more processing within the transaction
3. then do updates on the same table before ending the transaction

I am proceeding to add the 'UPDLOCK' and 'ROWLOCK' hints to the select statements. This seems safe enough to me, in the sense that as the DB normally operates at the default level of Read Commited, I don't think any functionality will be affected.

However...

I've now come to a case like in 1...2..3 above, where the original developer has added a (HOLDLOCK) hint to the select statement I was going to use (UPDLOCK, ROWLOCK) with.

Points go to the person who tells me IN PLAIN ENGLISH what the data integrity implications would be of changing the current

"select blah from blah WITH(HOLDLOCK)"
to
"select blah from blah WITH(UPDLOCK, HOLDLOCK)"
and to
"select blah from blah WITH(UPDLOCK, HOLDLOCK, ROWLOCK)"

in my transaction, bearing in mind there's loads of concurrency, with other users and apps reading and updating the same table constantly.

Thanxb4

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Asked On
2004-06-03 at 01:13:06ID21011629
Tags

updlock

Topic

MS SQL Server

Participating Experts
2
Points
0
Comments
7

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Answers

 

by: vc01778Posted on 2004-06-03 at 08:39:48ID: 11223825

1. "select blah from blah WITH(HOLDLOCK)"

The holdlock hint leaves shared locks  behind until the transaction is finished.  Normally,  the lock is removed as soon as the row/page was processed.

The updlock hint will block another select with the same hint because it puts an update lock instead of a shared lock so the concurrecy and pottential for dead-locking with this hint will be higher.  It won't block a statement with the holdlock+rowlock hints and vice versa.


2.  "select blah from blah WITH(UPDLOCK, HOLDLOCK, ROWLOCK)"

The rowlock hint will prevent (when possible) the lock escalation to the page/table level from happenning.  The concurrency will improve at the expense of memory consumed by the lock manager to maintain the row level locks.

VC

 

by: vc01778Posted on 2004-06-03 at 08:57:52ID: 11224036

Forgot to say...

HOLDLOCK, UPDLOCK will increase deadlock probability;  ROWLOCK will improve concurrency for the obvious reasons.

Also,  HOLDLOCK is the same as the SERIALIZABLE and it's unclear why you need UPDLOCK in addiditon to HOLDLOCK as they behave in the same way but UPDLOCK is stronger than HOLDLOCK.  You may want to use UPDLOCK if you want to reserve the row for a subsequent update and get rid of HOLDLOCK.

ROWLOCK is the only 'tool' to improve concurrency but before doing this you need to analyze and clearly understand your locking scenarios.


VC

 

by: jasonstephensPosted on 2004-06-09 at 15:41:10ID: 11274998

VC,
I'm confused by how UPDLOCK will increase deadlock probability.
If I use UPDLOCK with SELECT  in transaction A and transaction B to  "reserve the row for a subsequent update" doesn't that make it less likely that transaction A will select some rows from TableA and TableB (Shared lock), then process B will do the same, then each will try to get an exclusive lock on those tables and deadlock each other? Maybe I haven't understood UPDLOCK properly - or are there other concurrency issues in taking this approach?

 

by: vc01778Posted on 2004-06-09 at 16:10:04ID: 11275084

UPDLOC,  generally speaking,  will :

1. decrease concurrency:

with an UPDLOCK Session A's select will block on Session B's select, etc.  Without UPDLOCK,  no such blocking would occur.


2.  increase deadlock probability:

In the same table:

Session A selects row A
Session B selects row B
Session A selects row B
Session B selects row A
*deadlock*

VC

 

by: jasonstephensPosted on 2004-06-20 at 17:32:08ID: 11356184

In that case, how is UPDLOCK different from HOLDLOCK?

 

by: moduloPosted on 2004-07-30 at 07:31:02ID: 11677435

PAQed, with points refunded (500)

modulo
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