the select single is used retrive the single record from the
database. where as in the select single up to one row is
used retrive one row from the databse.

select up to one rows is much quicker that select single 
as your not really using the all the key fields. Select 
single is a construct that is designed primarily to read 
database records with a primary key. If there is no primary 
key available then at the most it might end up doing a 
perfunctory sequential search. However in the case of 
select up to one rows it will assume that there is no 
primary key and will automatically search for the most 
appropriate index.

In select * single only a single record is fetched that is 
whichever one will come first in the data base query. But 
in the case of select upto one rows all the records will be 
fetched and stored in a buffer which pertain to the where 
condition in the Data base theory.