Thursday, April 4, 2013

Avoiding a Common Deadlock

A common cause of deadlocks is concurrent identical update/delete queries.  For example, a query such as
DELETE FROM MyTable WHERE MyCode=@code
When two queries start executing this query with different codes, they have a good chance of deadlocking.  This can be via lock escalation, where query A requests an escalation to a table lock, which will wait on the query B.  As soon as the query B requests a lock escalation, or any lock that query A has, there is a deadlock.  You can have a deadlock without table escalation, by the two concurrent queries requesting page locks.  Eventually query A might ask for a page that query B has and query B is waiting on query A.
A very simple solution to this kind of deadlock is to take a Table lock at the beginning of the query.  That way whoever gets in first will finish before the other query starts, removing the possibility of a deadlock on that one table.  This can be as simple as
Taking a table lock will reduce parallelism.  No other queries (except NOLOCK) will be able to read the table while the TABLOCKX is held.   However, the reduction in parallelism is generally less painful that experiencing deadlocks.  Also, queries that DELETE/UPDATE many rows in a single query will necessarily lock large parts of the table (often the whole table if lock escalation is triggered), so taking an explicit table lock for the query generally is not too onerous.
Note, another trick that I sometimes use in stored procedures that are looping through many records with possible updates is to take an explicit lock on the table at the beginning of a transaction.  For example, the following code will take an explicit table lock on MyTable, which it holds until the transaction is committed.
-       - - put your code in here to perform updates/inserts/deletes on the table(s)
A couple of notes for deadlocks and locking
  • The amount of locking and deadlocks exponentially increase as transaction time increase.  So, anything you can do to reduce transaction time will reduce contention.
  • It is common practice for middle tier applications (eg .NET) to start transactions and call the database server multiple times within the transaction.  This is OK, but it is even safer (and reduced transaction time) if you encapsulate the logic in a stored procedure, and let the stored procedure begin/commit the transaction.
  • SELECT queries can be called without locks (NOLOCK).  DELETE/UPDATE/INSERT queries cannot be called without locking.
  • Read queries (DELETE/UPDATE/INSERT queries also read queries for this point) generally only lock the current record/page by using CURSOR STABILITY lock isolation.  It is possible for them to use more onerous strategies, such as, REPEATABLE READ, in which case they do not release read locks until the end of the transaction.  This can lead to contention and deadlocks with concurrent update queries. Sometimes these REPEATABLE READ read queries will benefit from an explicit table lock.
  • Lock escalation is generally your enemy in avoiding deadlocks.  If you see that a query is regularly escalating to a table lock, it is probably better for you to explicitly take a table lock at the beginning of the transaction/query as I outlined above.
  • Transactions can be started and committed while a stored procedure holds position on a cursor.  This way, you can commit and release locks every n records while processing rows in a cursor.
  • Use TRY CATCH in your procedures to handle exceptions such as deadlocks.

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