Simple query is executed very slow to 100 rows

Question

I use asp.net mvc, sql server. Query in my repository's class. Sometimes query is executed in 10 seconds, sometimes in 3 minutes!! Why? I used a SQL Server Profiler, but I realy don't understand what could be the cause and how I can find it.

Query:

SELECT 
    [Extent1].[Id] AS [Id], 
    [Extent1].[FirstAddressId] AS [FirstAddressId], 
    [Extent1].[SecondAddressId] AS [SecondAddressId], 
    [Extent1].[Distance] AS [Distance], 
    [Extent1].[JsonRoute] AS [JsonRoute]
    FROM [dbo].[AddressXAddressDistances] AS [Extent1]

Answer 1

Check your query plan. Just run your SELECT statement in SqlServer Management Studio to obtain real query plan. More info is here: Query plan.

If they are the same, but response time differs significantly between the calls, than probably the issue is with lockc on db level (or huge active workloads). I mean incorrect transaction isolation level for instance or some reports running in the meantime obtaining too much resources (or generating locks "because of something" to ensure some data consistency enforced by some developer). Many factors have influence on performance (including memory available at the moment of query execution).

You can run also a few queries to analyze quality of your statistics (or just update all of them using EXEC sp_updatestats) or please analyze fragmentation of the indexes. I guess, but by me locks and outdated stats or defragmented indexes, can force SqlServer to choose very inefficient query plan. Some info on active locks: Active locks on table

Additional info 1: If you are the only user of this db and it's on your local machine (you use SQLServer Express) the issue with locks is rather less possible then other problems. Try to open Event Log of SqlServer. It's available in SqlServer Management Studio on left side (tree) under your engine instance here: Management/Sql Server Logs/Current. Do you see any unusual info there? Try to review system log also (using Event Viewer app). In case of hardware problems you should see there also some info. Btw: how many rows do your have in the table? Try to review also behavior of your disks in some Process Explorer or Performance Monitor. If disk queue length is to big it can be main source of the problem (in such case look what apps stress disk)...

More info on locks:

    SELECT 
      [spid] = session_Id
    , ecid
    , [blockedBy] = blocking_session_id 
    , [database] = DB_NAME(sp.dbid)
    , [user] = nt_username
    , [status] = er.status
    , [wait] = wait_type
    , [current stmt] = 
        SUBSTRING (
            qt.text, 
            er.statement_start_offset/2,
            (CASE 
                WHEN er.statement_end_offset = -1 THEN DATALENGTH(qt.text)  
                ELSE er.statement_end_offset 
            END - er.statement_start_offset)/2)
    ,[current batch] = qt.text
    , reads
    , logical_reads
    , cpu
    , [time elapsed (ms)] = DATEDIFF(mi, start_time,getdate())
    , program = program_name
    , hostname
    --, nt_domain
    , start_time
    , qt.objectid
FROM sys.dm_exec_requests er
INNER JOIN sys.sysprocesses sp ON er.session_id = sp.spid
CROSS APPLY sys.dm_exec_sql_text(er.sql_handle)as qt
WHERE session_Id > 50              -- Ignore system spids.
AND session_Id NOT IN (@@SPID)     -- Ignore this current statement.
ORDER BY 1, 2
GO

Answer 2

Before you waste any more time on this, you should realize that something like the time a query takes in development is essentially meaningless. In development, you're running a single-threaded web server in IIS Express, which means that you've also got VS running, sitting on roughly 2-4 GB of RAM. Together with that, you're running a SQL Server instance, that's fighting the system for both RAM and hard drive time. You haven't given any specs of your system, but if you also happen to be sporting a consumer-class 5400 or 7200 RPM platter-style drive rather than an SSD, that's going to severely impact performance as well. Then, we haven't even got into what else might be running on this system. Photoshop? Outlook? Your favorite playlists of MP3s decoding in the background? What's Windows doing? It might be downloading/applying updates, indexing your drive for search, etc. None of that applies any more when you move into production (or at least shouldn't). In production, you should have a dedicated server with 4-8 GB of RAM and an SSD or enterprise-class 15,000+ RPM platter drive devoted just to SQL Server, so it can spit out query results at lightning speeds.

Long and short, if you want to guage website/query performance of your application, you need to deploy it to a facsimile of what you'll be running in production. There, you can pound the hell out of it and get some real data you can actually do something with. Trying to profile your app in development is just a total waste of time.