SQL Server 2005的奇怪查询性能

Question

I noticed some performance problems with my DB. Such query (just for a example):

SELECT * 
  FROM ActionHistory 
 WHERE ObjectId = @id"  

...executes at random with different reads and duration. ObjectId is Foreign Key, with index on it.

With SQL Profiler I found, that sometimes the results are: 5 reads, 0 duration, but in another case: 5 reads, 200 duration. Such big durations occurs accidentally.

I use distributed transaction with WCF. Such results I got when I was the only user at that time, so it likely not to be a locks or something else.

What is the reason of such behaviour: low reads, but high query duration ?

Answer 1

In general, distributed transactions are extremely expensive. Try disabling distributed transactions in your environment to see if that changes anything.

Answer 2

Since the query is exactly the same each time and the reads are the same, then it's most likely due to locking. Sometimes another query is executing and may have a lock on the records that need to be accessed. Waiting for the lock to be released would cause a slowdown.

Using SQL Profiler to compare start/stop times for queries you can identify overlapping queries that may cause locking.

This is not an indication of a problem, just an explanation of the differences you're seeing.

Answer 3

Enable read committed snapshot in the database:

ALTER DATABASE ... SET READ_COMMITTED_SNAPSHOT ON;

This will miraculously change your reads that occur under the default read-committed isolation into snapshot reads, which are not hindered by locks. See Choosing Row Versioning-based Isolation Levels for details, including the runtime resource usage caused by enabling snapshot reads.