Now that we know that footprint is the biggest cause of deadlocks, how do we fix it? By minimizing the number of locks and the duration each of these locks is held. Once an isolation level like this is set, no two types of locks will be compatible, greatly increasing the chances for a deadlock. It is possible to have rogue application connections come into the database and request a very restrictive isolation level (say serializable). Transaction Isolation is important because it is set at a connection level. You always want to perform the same operations against a table in the same order. If transaction 1 updates before it selects, and transaction 2 selects before it updates, then the chances of these two transactions colliding is greatly increased. Let’s say you have two separate transactions that both update and select records from the same table. I try to avoid explicit transactions as much as possible for this reason. Transaction order applies if you utilize explicit transactions (BEGIN TRAN. Maybe the the procedure can run as a scheduled job rather than being user-invoked. If you can’t do that then you may need to look at a design change. In these cases your best bet is to try and decrease the footprint. You can have colliding queries that have a relatively low footprint, however because they run so frequently, the chances of them colliding are much greater. Most slow queries have a large footprint by means of both locks and duration.Ĭoincidence is the frequency in which the colliding queries are run. Some queries can acquire a lot of locks yet be done in a short period of time. By footprint I mean the amount of locks and duration those locks are held by a query. By far the biggest offender I’ve seen on most systems is the footprint issue. Troubleshootingįour things come to mind when deadlocks occur. But if the only color each of them can reach is a spot that each of them already occupy, then they are deadlocked. Each person represents a transaction and each transaction needs a certain color circle to finish their goal. To understand this you can think of the game twister. Once completed user 2 will only ever see the result of a completed update rather than a half completed update.Ī deadlock happens when two transactions each grab one resource, then the second resource each transaction needs, is the first resource that each of them grabbed. With locks, User 2 would have to wait in line until the update is completed. Without locks, user 2 may see half of the records inactivated, while the other half is activated. Imagine user 1 begins updating a table to inactivate all the records while user 2 queries this table in the middle of the operation. Locks are acquired so that users have a consistent view of what the data looks like at the time they are querying it. Each of these types of locks may or may not be compatible with other types of locks. There are many different kinds of locks and many different objects that can get locked. So with read-committed we have less tempdb data swapping, but more locking. So if you are ever considering enabling snapshot isolation as your default isolation level for SQL Server you will want to make sure you have a beefy RAID 10 tempdb storage. Snapshot isolation is much more disk intensive because these “snapshots” need to be persisted in a temporary data store. There are trade-offs with shapshot however. Now what is read committed? Some people are critical of this isolation level because with a different isolation level like Snapshot, it is much less likely that deadlocks occur because each transaction utilizes it’s own version of what a table looks like at the very time the transaction begins. I’ve gotten some push back sometimes from readers when I write articles that do not give cookie cutter solutions, but the “teach a man to fish” adage couldn’t be more true for deadlocks.Īll of it revolves around locking, which is a central piece to SQL Server’s default transaction isolation level of read-committed. In order to fix deadlocks, it’s critical to understand why they occur.
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