Which SQL Server Method for Truncating Datetime Components Offers Better Performance: DATEADD/DATEDIFF or CAST/CONVERT?

Truncating the time part of datetime fields in SQL Server: Performance Analysis

When dealing with datetimes in SQL Server, it is often necessary to truncate the time portion. Two common ways to achieve this are:

1. DATEADD(DD, DATEDIFF(DD, 0, GETDATE()), 0)
2. CAST(CONVERT(CHAR(11), GETDATE(), 113) AS DATETIME)

While both methods appear to be fast, the question is: which method performs better, especially when dealing with large data sets?

Comparison of methods

Testing with a hypothetical dataset of one million rows shows that approach (a) uses slightly less CPU than approach (b). This suggests that DATEADD/DATEDIFF is the preferred way to truncate the time part.

Advantages of DATEADD/DATEDIFF

In addition to performance considerations, DATEADD/DATEDIFF also provides the following advantages:

• Avoid potential language/date format issues related to VARCHAR.
• Does not rely on internal storage such as FLOAT.
• By modifying the "0" base, it can be extended to calculate the first day of the month, tomorrow and other time periods.

SQL Server 2008 Alternatives

In SQL Server 2008 and later, you can use the CAST function to convert directly to the DATE data type, effectively truncating the time portion. Alternatively, you can simply use the DATE data type from the beginning to avoid the need for any truncation.

Performance optimization

It should be noted that if you use functions or casts in the WHERE clause, you may invalidate the index. Therefore, it is important to carefully consider the optimization implications before using these methods in these situations.

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