Stephen A. Fuqua (SAF) is a Bahá'í, software developer, and conservation and interfaith advocate in the DFW area of Texas.

(SQL) Blocking Giving You the Blues?

October 5, 2008

Problem: You have a long-running query in SQL Server that is causing failures all over the place. Activity Monitor shows you the cause is blocking.

Causes: Blocking essentially means "you’ve locked a table, and now someone else is stuck waiting for you to unlock it." We all know (or should know) that transactions cause table locks, and thus blocking. That’s one reason that transactions need to be short and sweet. But there is another source of blocking, one that can be a bit more subtle: long-running queries that are trying to read committed data only.

READ COMMITTED is the default isolation level for SQL Server 2005 (and most other versions, I presume). Ben Gan describes: "In this isolation level, processes request a shared lock to read data and release it as soon as the data has been read – not when the transaction terminates." Thus if you have a poorly-performing SELECT query, it might take a while before the shared lock is released. In the meantime, everyone else is blocked.

Solutions: If transaction management is your problem, then you need to re-evaluate the strategy. Small transactions are needed. You might even need to fake a transaction. Real SQL gurus might have a better strategy, but one that I have used is to mark a record with an error code of "TR" for transaction. For instance, I have two tables, one that somewhat summarizes the second. I insert into the first, and then insert thousands of records into the second one.

Originally I thought to rollback the whole transaction if any of the thousands of inserts failed. But I realized that would be impractical. So instead I marked the parent record with "TR", and when done with the thousands I update that record to remove the TR. At any time I can see that this particular record is not yet finished, or I can create a custom rollback strategy that just deletes the record.

If a long-running query is the problem, then you might consider turning off locking by setting the isolation level to READ UNCOMMITTED. But don’t do this blindly – you’ll want to carefully evaluate the tables you are querying. What is the likelihood that some of your data will change during the lifetime of the query? If there is any meaningful chance of change, then you’ll also have a meaningful chance of reading "dirty data." But, if your tables are rarely updated, or you are querying only "old" data that won’t be touched, perhaps this is a safe maneuver. Worth a try anyway :-).

Finally, you really ought to look at the root cause – why is the query taking so long? Do some performance tuning: create new indexes that eliminate table scans, key lookups, and hash matches, for instance.

PS. my suggestion may be a ticking timebomb; also consider SNAPSHOT isolation level.