Thursday, June 10, 2010

Bucket DateTimes in T-SQL

Today at work I was working on a SQL Server Reporting Services report which shows us the number of rows being created in a certain table in each 10-minute interval. While working on this, I tried several different ways in which I could "bucket" the datetime column into the same ten-minute period. At the end, I finally decided to extract this logic into a User-Defined Function which can bucket any datetime into any size interval.

Here's the function:

/* BucketDateTime:
* Buckets any datetime into an interval of the given size.
* Returns the minimum datetime for the interval chosen, or NULL if @MinutesPerBucket < 1
* Example: bucket times 7:29pm and 7:31pm to the nearest 30 minute interval
* SELECT dbo.BucketDateTime('2010-04-01 19:29:00', 30) => '2010-04-01 19:00:00.000'
* SELECT dbo.BucketDateTime('2010-04-01 19:31:00', 30) => '2010-04-01 19:30:00.000'
CREATE FUNCTION [dbo].[BucketDateTime]
@SourceDateTime DATETIME, -- The DateTime to bucket
@MinutesPerBucket INT -- The number of minutes per bucket.

IF (@MinutesPerBucket < 1) RETURN NULL

-- The earliest possible datetime

-- # of minutes passed between the epoch and source datetime
DECLARE @MinutesSinceEpoch INT

-- The value to bucket @SourceDateTime into, based on @MinutesPerBucket

-- # of minutes passed between @BucketDateTime
DECLARE @BucketMinutesSinceEpoch INT

-- This value is from Books online:
SET @Epoch = CAST('1753-01-01' AS DATETIME)

SET @MinutesSinceEpoch = DATEDIFF(mi, @Epoch, @SourceDateTime)

SET @BucketMinutesSinceEpoch = @MinutesSinceEpoch - (@MinutesSinceEpoch % @MinutesPerBucket)

SET @BucketDateTime = DATEADD(mi, @BucketMinutesSinceEpoch, @Epoch)

RETURN @BucketDateTime



Tuesday, June 8, 2010

Execute LINQ queries without locking any tables

At work I'm building an application which queries a SQL Server backend with LINQ to SQL, and I recently found that some of my queries were slow due to table locking. I found this excellent blog post referencing Scott Hanselman's blog post which describes how to execute LINQ queries without locking the source tables:

Deciding to go with option 1, I came up with an elegant solution for easily adding this to any LINQ queries. Let's say you are using a LINQ query like this:

var query = from f in Foo
select new { Foo = f };

foreach (var item in query)
//Do something

In this case, the query is actually executed in the foreach loop. By implementing an extension method, you can easily change this to use a transaction with the "Read Uncommitted" Isolation Level like so:

var query = from f in Foo
select new { Foo = f };

foreach (var item in query.ToListNoLock())
//Do something

The extension method is implemented like so:

public static class IEnumerableExtension
public static TransactionScope CreateNoLockTransaction()
var options = new TransactionOptions
IsolationLevel = IsolationLevel.ReadUncommitted
return new TransactionScope(TransactionScopeOption.Required, options);

public static List<T> ToListNoLock<T>(this IEnumerable<T> query)
using (TransactionScope ts = CreateNoLockTransaction())
return query.ToList();

And then of course you can implement new versions of various other extension methods on IEnumerable/IQueryable, such as SingleOrDefault, FirstOrDefault, etc.

Note that this is the same as adding (NOLOCK) hints to EVERY table in your query. I do not currently know of a way to add nolock hints to individual tables.