Why does the datetime type in SQL Server Compact round values?

Question

I've run across something that surprised me.

I am using WinPcap to collect data off a network. Internally, WinPcap uses the Windows performance counters to generate its timestamps. I know they are subject to drift, but these timestamps nevertheless have precision down to the microsecond level.

If I insert these timestamps into a SQL Server Compact 4.0 database as a datetime value and extract them later, I noticed that the precision has dropped to milliseconds.

For example,

10:52:19.706084 -> 10:52:19.706000

Now, I have since read here that SQL Server rounds values with the datetime type to .000, .003, or .007 milliseconds. That explains what is happening.

Now, the datetime field uses 8 bytes to store its data, 4 bytes for the date and 4 for the milliseconds since midnight. But if I call DateTime.ToBinary(), I get back an 8-byte number that represents the value in all of its precision. In fact, if I write this value to the database in a bigint column and then call DateTime.FromBinary() when extracting that value, I get the original value with the same precision.

This is the approach I'm going to use, but I'm still curious: Why didn't the original datetime type in SQL Server Compact use DateTime's ToBinary/FromBinary storage mechanism?

EDIT:

As Aaron Bertrand rightly points out, SQL Compact does not support datetime2. Further, datetime2 uses 6, 7, or 8 bytes, not 54 bytes in regular SQL Server. My basic question still stands, though.

Answer 1

I don't know the full internal details or the motivation behind the choice, but datetime is stored internally as - essentially - two 4-byte integers. One represents date, the other represents time. I suspect you lose some precision in the latter because of the way ticks / milliseconds have been handled since the very first versions of SQL Server, but again, I don't know low-level implementation details.

Related questions for more background info:

• What is the internal representation of datetime in sql server?

• Allow Entity Framework 4.5 to use datetime2 with SQL Server CE4

In order to support the precision you want without moving the value in and out of binary format, I would suggest using LocalDB which has the same portability advantages of Compact but without many of the feature limitations (such as support for the more precise datetime2 type - which I assure you takes 6-8 bytes, not 54 :-)).