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What is determinism?

Seeder question: I have read in BOL that some functions are deterministic, and some functions are non-deterministic? What does this mean and what is an example of each? Are there any functions that are both?

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asked Oct 16, 2009 at 07:05 PM in Default

Matt Whitfield gravatar image

Matt Whitfield ♦♦
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Great question.
Oct 16, 2009 at 08:32 PM RBarryYoung
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3 answers: sort voted first

A deterministic function is one where you can be sure of the output based on input (in a particular database). So if you call the DAY function on a particular field, it will return you the numerical day for the date parameter passed in. DAY( '1-1-2001') always returns 1.

A non-deterministic function is one that you can't predict the output. So RAND (with no seed) is a good example, but GETDATE(), @@CPU_BUSY, and similar functions that can't be predicted based on the input.

Some functions can be either, depending on how they are used. Here's a good reference from Books Online: Deterministic and Nondeterministic Functions

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answered Oct 16, 2009 at 07:35 PM

Steve Jones - Editor gravatar image

Steve Jones - Editor ♦♦
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How did I guess you might have the best answer Steve? :)
Oct 16, 2009 at 09:03 PM Matt Whitfield ♦♦
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A deterministic function will always return the same result given the same arguments, non-deterministic functions will vary each time they are called. For example a function that adds 1 to the argument passed in would always return 8, if the argument was 7 - a deterministic function. On the other hand, the GETDATE() Function returns a different value each time called, so it's non-deterministic.

http://msdn.microsoft.com/en-us/library/ms178091.aspx

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answered Oct 16, 2009 at 07:33 PM

Jay Bonk gravatar image

Jay Bonk
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A function can't be both, but GETDATE() is a function that will be treated as ok to use in user-defined functions because the system gets what GETDATE() does. It doesn't change with system settings, session options, etc.

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answered Oct 16, 2009 at 07:57 PM

Rob Farley gravatar image

Rob Farley
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A function most certainly can be both - CONVERT being the most famous example...
Oct 16, 2009 at 09:01 PM Matt Whitfield ♦♦
Ok, but a function called in a certain way is either one or the other. Rand() with a seed is always deterministic. Without it, it never is.
Oct 16, 2009 at 09:20 PM Rob Farley
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asked: Oct 16, 2009 at 07:05 PM

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Last Updated: Oct 16, 2009 at 07:05 PM