Can function in join reduce the performace.

Question

hi,
   i have a function fn_oneortworow it accepts one parameter.
i have used it in left join as follows
1)
select * from T
left join fn_oneortworow(@a) a on a.id=t.id
my fear is that it may execute as many times as many rows are there in table "T"
so i have made the query as follows
2)
select * into #temp from join fn_oneortworow(@a)
select * from T
left join #temp a on a.id=t.id
Q1)please tel me that is it correct that in first query function will called as many times as many records are ther in table T?
Q2)and is second formulation is good or should i use the first one. (to imporve the performance)
yours sincerely

Answer 1

It probably will call the function only once.  Since you are always calling it with the same parameter value, SQL knows the the function will always return the same result.  It is easy to check by looking at the estimated execution plan, that will
tell you how many times SQL believes it will be called.
Nevertheless, even if it is only called once, I would still do the temp table and load the table with a function call.  Then you know for sure the function is only called once.
In addition, if the function can return a lot of rows and id can be a primary key to to #temp table, I would explicitly declare the #temp table with id as a primary key.  That will make the join much faster if #temp has many rows.  And if id is
not a suitable primary key, but #temp has lots of rows, I would create an index on id to speed up the join.
But the only way to know for sure is to run the possible choices with a realistic set of data and see which method is best.
Tom