It depends on the functions and, in some cases, the number of records in your database. Simple functions like Get ( currentHost ) or Get ( CurrentDate ) may execute so quickly that you cannot tell the difference unless this is in a loop cycling through 1000's of records or some such.
In which case, the variable might be the faster alternative.
This is definitely true for aggregate functions such as Sum, Average, Std Dev. If they are used to produce an aggregate value from 1000's or hundreds of 1000's of related records, then computing the value once will clearly be faster.