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    <p>I must admit I hadn't seen the floor-float conversion shown by Matt before. I had to test this out. </p> <p>I tested a pure select (which will return Date and Time, and is not what we want), the reigning solution here (floor-float), a common 'naive' one mentioned here (stringconvert) and the one mentioned here that I was using (as I thought it was the fastest).</p> <p>I tested the queries on a test-server MS SQL Server 2005 running on a Win 2003 SP2 Server with a Xeon 3GHz CPU running on max memory (32 bit, so that's about 3.5 Gb). It's night where I am so the machine is idling along at almost no load. I've got it all to myself.</p> <p>Here's the log from my test-run selecting from a large table containing timestamps varying down to the millisecond level. This particular dataset includes dates ranging over 2.5 years. The table itself has over 130 million rows, so that's why I restrict to the top million.</p> <pre><code>SELECT TOP 1000000 CRETS FROM tblMeasureLogv2 SELECT TOP 1000000 CAST(FLOOR(CAST(CRETS AS FLOAT)) AS DATETIME) FROM tblMeasureLogv2 SELECT TOP 1000000 CONVERT(DATETIME, CONVERT(VARCHAR(10), CRETS, 120) , 120) FROM tblMeasureLogv2 SELECT TOP 1000000 DATEADD(DAY, DATEDIFF(DAY, 0, CRETS), 0) FROM tblMeasureLogv2 </code></pre> <blockquote> <p>SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 1 ms.</p> <p>(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.</p> <p>SQL Server Execution Times: CPU time = 422 ms, elapsed time = 33803 ms.</p> <p>(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.</p> <p>SQL Server Execution Times: CPU time = 625 ms, elapsed time = 33545 ms.</p> <p>(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.</p> <p>SQL Server Execution Times: CPU time = 1953 ms, elapsed time = 33843 ms.</p> <p>(1000000 row(s) affected) Table 'tblMeasureLogv2'. Scan count 1, logical reads 4752, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.</p> <p>SQL Server Execution Times: CPU time = 531 ms, elapsed time = 33440 ms. SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 1 ms.</p> <p>SQL Server Execution Times: CPU time = 0 ms, elapsed time = 1 ms.</p> </blockquote> <p>What are we seeing here?</p> <p>Let's focus on the CPU time (we're looking at conversion), and we can see that we have the following numbers:</p> <pre><code>Pure-Select: 422 Floor-cast: 625 String-conv: 1953 DateAdd: 531 </code></pre> <p>From this it looks to me like the DateAdd (at least in this particular case) is slightly faster than the floor-cast method.</p> <p>Before you go there, I ran this test several times, with the order of the queries changed, same-ish results.</p> <p>Is this something strange on my server, or what?</p>
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    1. COIt's not strange at all. First, in my opinion converting to float is poor practice, because a round-trip conversion to datetime is not reliable. Second, please see [this post with more performance testing of the various methods](http://stackoverflow.com/questions/2775/whats-the-best-way-to-remove-the-time-portion-of-a-datetime-value-sql-server/3696991#3696991).
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