I have an app that is run as a service in production, but we do some manual testing with a simple GUI - practically nothing going on in it, it's just a wrapper with a textbox for input.
I recently changed my database schema and updated my mappings to match, and then the GUI suddenly worked very slowly on a trivial case. After some logging and running it multiple times, it turned out the new bottleneck was this query:
public void Refresh()
{
using (var session = _sessionFactory.OpenSession())
{
_allFields = session.Query<FieldDefinition>().ToList();
}
}
Repeatedly, it was taking 1:08 minutes for that one method (even though there's only about 300 FieldDefinitions in the database). By this time I was tired of manually re-running the GUI, so I wrote a unit test that exercised the exact same case - but I couldn't reproduce the slowdown.
My test called the same top-level object that the GUI does with the same input. I would expect this to run in nearly identical amounts of time. But when run in Visual Studio with MSTest, that same query was taking less than two seconds. That's 1/30th the time. It was doing the exact same work, just significantly faster.
Things I checked to see if I could make them run the same:
- Both approaches produced the same number of SQL statements.
- It doesn't seem to be caused by the JITter (multiple runs of the GUI without restarting it, same time results over and over)
- Isolating it so that it used a brand new
ISessionFactory for each Refresh had no effect
- Turning off logging (log4net) had no effect
Changing the query to eager-load a child did work... kind of: After applying the fix, the WinForms app is only as fast as the unit test already was. The speed of the unit test did not significantly change (tenths of a second).
The old query was causing a select n+1 issue: but the issue was present in both the Winform and MSTest runs. Only the WinForm app saw a significant slowdown because of it.
How can I explain this? Why would only the WinForm app suffer a massive slowdown during a Select N+1 query?