What's the Most Efficient SQL Server Method to Extract the Date from a DateTime Value?

The best way to efficiently extract dates from datetime values ​​in SQL Server

In MS SQL 2000 or 2005, for a datetime value like '2008-09-25 12:34:56', the most efficient way to get a datetime value containing only '2008-09-25' has been is a topic of debate. While various methods have been proposed, a comprehensive analysis reveals the following results:

Test environment and methods:

Performance comparison was performed on a test server (MS SQL Server 2005) containing a large table containing timestamps with millisecond precision. The table spans over 2.5 years and contains 130 million records, with the first million records selected for analysis.

Query for evaluation:

• Pure SELECT statement
• floor-float conversion (CAST(FLOOR(CAST(CRETS AS FLOAT)) AS DATETIME))
• String conversion (CONVERT(DATETIME, CONVERT(VARCHAR(10), CRETS, 120), 120))
• DATEADD conversion (DATEADD(DAY, DATEDIFF(DAY, 0, CRETS), 0))

Result:

CPU time (ms):

• Pure SELECT statement: 422
• floor-float conversion: 625
• String conversion: 1953
• DATEADD conversion: 531

Conclusion:

Contrary to previous claims that the floor-float method is fastest, it turns out that the DATEADD conversion emerges as the most efficient method in this particular scenario.

Note: It is important to note that these results may vary based on specific server configuration and dataset characteristics.

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