When I update/insert a single row should it lock the entire table?

Question

I have two long running queries that are both on transactions and access the same table but completely separate rows in those tables. These queries also perform some update and inserts based on those queries.

It appears that when these run concurrently that they encounter a lock of some kind and it’s preventing the task from finishing and locks up when it goes to update one of the rows. I’m using an exclusive row lock on the rows being read and the lock that shows up on the process is a lck_m_ix lock.

Two questions:

1. When I update/insert a single row does it lock the entire table?
2. What can be done to work around this sort of issue?

Answer 1

Typically no, but it depends (most often used answer for SQL Server!)

SQL Server will have to lock the data involved in a transaction in some way. It has to lock the data in the table itself, and the data any affected indexes, while you perform a modification. In order to improve concurrency, there are several "granularities" of locking that the server might decide to use, in order to allow multiple processes to run: row locks, page locks, and table locks are common (there are more). Which scale of locking is in play depends on how the server decides to execute a given update. Complicating things, there are also classifications of locks like shared, exclusive, and intent exclusive, that control whether the locked object can be read and/or modified.

It's been my experience that SQL Server mainly uses page locks for changes to small portions of tables, and past some threshold will automatically escalate to a table lock, if a larger portion of a table seems (from stats) to be affected by an update or delete. The idea is that it is faster to lock a table (one lock) than obtaining and managing thousands of individual row or page locks for a big update.

To see what is happening in your specific case, you'd need to look at the query logic and, while your stuff is running, examine the locking/blocking conditions in sys.dm_tran_locks, sys.dm_os_waiting_tasks or other DMV's. You would want to discover what exactly is getting locked by what step in each of your processes, to discover why one is blocking the other.

Answer 2

The short version:

1. No
2. Fix your code.

The long version:

LCK_M_IX is an intent lock, meaning the operation will place an X lock on a subordinate element. Eg. When updating a row in a table, the operation table takes an IX lock on the table before locking X the row being updated/inserted/deleted. Intent locks are common strategy to deal with hierarchies, like table/page/row, because the lock manager cannot understand the physical structure of resources requested to be locked (ie. it cannot know that an X-lock on page P1 is incompatible with an S-lock on row R1 because R1 is contained in P1). For more details, see Lock Modes.

The fact that you are seeing contention on intent locks means you are trying to obtain high level object locks, like table locks. You will need to analyze your source code for the request being blocked (the one requesting the lock incompatible with LCK_M_IX) and remove the cause of the object level lock request. What that means will depend on your source code, I cannot know what you're doing there. My guess is that you use an erroneous lock hint.

A more general approach is to rely on SNAPSHOT ISOLATION. But this, most likely, will not solve the problem you're seeing, since snapshot isolation can only benefit row level contention issues, not applications that request table locks.

Answer 3

A frequent aim of using transactions: keep them as short and sweet as possible. I get the sense from your wording in the question that you are opening a transaction, then doing all kinds of things, some of which take a long time. Then expecting multiple users to be able to run this same code concurrently. Unfortunately, if you perform an insert at the beginning of that set of code, then do 40 other things before committing or rolling back, it is possible that that insert will block everyone else from running the same type of insert, essentially turning your operation from free-for-all to serial.

Find out what each query is doing, and if you are getting lock escalations that you wouldn't expect. Just because you say WITH (ROWLOCK) on a query doesn't mean SQL Server will be able to comply... if you are touched multiple indexes, indexed views, persisted computed columns etc. then there are all kinds of reasons why your rowlock may not hold any water. You also might have things later in the transaction that are taking longer than you think, and maybe you don't realize that the locks on all of the objects involved in the transaction (not just the statement that is currently running) can be held for the duration of the transaction.