I am trying to debug a web app that uses a database and I need a little help. The problem is that part of the code throws a timeout error. I started using SQL Server Profiler
to check the communication between the app and the database. (I am not really a database expert and this is a new tool for me).
The app is supposed to call an UPDATE
on a specific record. Shortly after, it calls a SELECT
on the same record (because it refreshes the screen). I can see both the UPDATE
and the SELECT
event classes in the Profiler trace. (There are a few other event classes between them). The SELECT
is showing with a duration of 30499 and an Error of 2 - Abort
. I am guessing this is the source of my timeout error.
This is where it gets interesting. By selecting the EventClass
in the Profiler trace, I can see the SQL SELECT
statement that is being called. I copied and pasted this into a new query in Management Studio and tried it out. The SELECT
statement worked fine and returned the record before my finger left the f5 key. However, the data in the record was the old data. It seems that the previous UPDATE
call did not change the record.
Looking at the Profiler trace again, the UPDATE
event class shows an Error of 0 - OK
. When I select the UPDATE
event class and copy the SQL code into Management Studio it runs fine and does update the record. Is anyone able to help me work out what is going on please.
- If the
UPDATE
statement is shown in Profiler with no error, then does this mean that it should have updated the database? - What could be causing the
SELECT
statement to abort (considering that it runs fine when I try it in SQL Server Management Studio)? - Could problems with the
UPDATE
be causing the record to lock up which stop theSELECT
from running?
Any advice to help me debug and fix this would be appreciated.
Best Answer
Yes, but whether the change is visible to other database connections depends on whether the change is committed or not, and on the current transaction isolation level in use by the other connection. To be clear, the SQL:BatchCompleted Profiler Event is triggered when a T-SQL batch completes, it does not mean that any containing transaction has also committed.
In your case, the update was performed inside a transaction which was not committed. The
SELECT
statement was issued on a separate database connection under an isolation level that does not permit reading uncommitted data. From the description given, theSELECT
was blocked by the exclusive lock held by the uncommitted change.The fact that you were able to issue the
SELECT
and have it return the pre-update version of the row means your Management Studio session was using a row-versioning isolation level, almost certainlyREAD COMMITTED SNAPSHOT ISOLATION
.The
SELECT
issued from the application must be running under a different isolation level, one that does not use row-versioning, and blocks when it encounters an exclusive lock, perhapsSERIALIZABLE
. You should check the application code to determine this.There are lots of options here. You might find also tracing the SQLTransaction Event Class sufficient for your needs. Alternatively, there are many system views and commands to help you see uncommitted transactions. You may find a query like the following useful to show details for open user transactions:
The system views used in that code are all documented in Books Online:
sys.dm_exec_sessions
sys.dm_tran_session_transactions
sys.dm_tran_database_transactions
You may also find
DBCC OPENTRAN
useful as a way to quickly see the longest open transaction.